Vertical Stringing Testing - Part II

[tac-40]

I too completed modifications and testing to eliminate my vertical stringing in my 16.5" barrelled Century G1. Kotengu took a different approach as outlined in the original thread and his results from testing located HERE (http://www.falfiles.com/forums/showthread.php?s=&threadid=108858) and the original thread located HERE (http://www.falfiles.com/forums/showthread.php?s=&threadid=105291).

I went a similar route, but only focused on one thing. I had originally tried to fit up a buffer under the bolt lug just in front of the locking shoulder. During the fit-up phase I was able to get a good Bolt and B/C lockup with minimal movement. Plus everything appeared to lock back into the same spot each time the bolt was cycled. No firing test was done on this because I developed some concerns for reliability and safety, as well as wanting a permanent fix and not something that would last a certain number of cycles. My concerns about reliability were due to the positioning of the bolt lug slightly higher on the L/S. If there were some unwanted rearward movement of the bolt, it could disengage prematurely from the L/S and cause a catastrophic failure. And because I had limited resources available to me, I could not modify the B/C like HebrewBattleRifle did.

What I had were several bolts in good condition. I decided to weld a pad onto the rear portion of the bolt in order to force the bolt downward into the locking shoulder. My goal was to have the bottom of the bolt to contact the top flat of the L/S for maximum engagement and repatability. I used a TIG welder to lay a bead in the wear area of the bolt. I worked in steps to minimize the heat put into the bolt.

After initial hand filing to eastablish the needed profile, I began fitting the bolt to the receiver. This was done by filing the pad down until the Bolt an B/C would close with light thumb pressure, similar to headspacing. Once that was determined, I used a GO gauge to simulate a cartridge and continued to fit-up the Bolt and B/C combo until it would close the GO gauge, again using light thumb pressure. Double checked with NO-GO to verify headspace still good.

After verifying that the action would cycle dummy ammo, went to the range for testing. First off, had to adjust gas to compensate for tighter action. Needed about 3 clicks. Went from a 7 to a 4. Oh yeah, the gas port is drilled to .125 on this rifle with stock gas system components. No signs of vertical stringing. Groups were in the 4.5 to 5 inch range and had the occasional flyer. I believe the group size and flyers were due to the "trigger nut interface". Plus, the heavy stock trigger pull is not conducive to accuracy.

All in all, I am satisfied with the outcome. I now have a rifle that will keep all shots "in the black" at 100 yds and fuctioned with 100% reliability. The vertical stringing is gone.

[tac-40]

The orginal bolt and its dimensions

[tac-40]

Area to be welded

[tac-40]

Bolt after fit-up. File chatter marks left as wear indicators. Thats my story and I sticking to it.

[tac-40]

Final dimensions after fit-up.

[tac-40]

Best group of the day using Aussie at 110 yds measured with Bushnell Laser Range finder. Like I said, I am happy with the results. Horizontal displacement due to shooter and heavy trigger pull, I hope.

[kotengu]

Good thread, and congrats on the results! Any idea of the hardness of the weld material? I'm curious to see how it wears......

I'm surprised that .004" made that much of a difference, but you can't argue with results - well done!

[tac-40]

Any idea of the hardness of the weld material? I'm curious to see how it wears......

Unfortunately, I don't have any idea. I went to a local fabricator shop that had been recommended to me and told them what I was doing. I asked for their recommendation on what filler rod to use and they gave me one of theirs. I was told that it was what they used on moving parts and that it was the next best thing to Tungsten. Of course he laughed after saying that. And I didn't write down the type for this documentation, it figures.

I am hoping to get several thousand rounds out of the fix. Keeping it well lubed and the fact that I don't do prolonged shooting should minimize the wear. I wouldn't have done this fix to a full auto weapon because of the potential wear.

[fastfreddy]

How much vertical stringing did you have before the modification?

[robmac]

Tac-40 - you are the man!!:bow:

[Deltaten]

tac-40:

Kudos to you and kotengu. I think the mods, and the associated improvement,
are the first real-world fix for the stringing problem. Much appreciated.

Best to you both,
Paul

[supersniper]

That is the fix that I used to cure my problems. Now the only low shot is first round which is caused by the extra mass of the charging handle, working on a solution to it now.

By the way, my groups after the first round are in the range of 1/2 to 3/4 inch.

Not bad for a FAL huh?

Yes my friends, the FAL can be turned into a viable precision rifle.

Shoot Straight, :fal:

[dougjones31]

Changing your bolt/carrier combinations to get one that has no "slop" is the easiest fix. Usually only takes 3 bolts and carriers to get a winning combo.

There is just no reason to do this mod. Unless you just feel like welding and using a file.

[tac-40]

Changing your bolt/carrier combinations to get one that has no "slop" is the easiest fix. Usually only takes 3 bolts and carriers to get a winning combo.

There is just no reason to do this mod. Unless you just feel like welding and using a file.

Do not think that I rushed into the weld mod to correct the stringing issues with my rifle. I researched very carefully both the relationship of the parts and materials and what could be the root cause of the problem.I tried your method first, because I am a firm believer of minimal changes of existing designs. I had 6 carriers and 5 bolts from which to mix and match. I could not, repeat, could not cure the stringing issues. Yes, I saw some improvement, but not enough to be acceptable. So I chose to modify the bolt in the manner that I did based upon the equipment at hand and my skill levels. Kotengu used another method that I truly like, but , alas, was unable to perform because of the equipment availability.

As of now, I have identified a couple of other things I want to try in order to wring out the best accuracy the rifle can give. I am happy with the performance now, but feel it can be better. So begins a long term project on improving the accuracy of this particular rifle.

[dougjones31]

Your problem must have been big, but I am sure that more choices of carriers and bolt would have allowed you to find a perfect pair.

I too fixed my first stringing rifle with the screw in the carrier method. After I had completed the mod, the stringing went away. A little time passed and I tried replacing the carrier/bolt with a hand matched pair and that allowed no stringing either.

After that , I realized that the mod is not needed.

[kotengu]

Speaking of the set-screw mod -

I might as well add here that the calculations of the thread-root stresses (the high stress point in the set screws) are borderline in the worst-case scenario. In other words, I would NOT at this time give it an OK to do this mod as a production run if I were the approving engineer. It's probably OK, but the "infinite life" numbers are too close without a comfortable factor of safety to give it the full go-ahead. It is plenty safe for a single shot, but if one were to experience multiple case-head separations it MAY fail, sending the set screw shooting out the top and through the topcover, MAYBE sending the topcover your way in the form of shrapnel. The nice thing about this is that the carrier is plenty strong to still be used, even with the empty hole, provided you survive the set-screw launching :D

That being said, I've done it to mine and will watch how it wears, wearing my safety glasses the entire time........

but if anybody reading this is foolish enough to do it too and ends up with a topcover in their forehead don't say I didn't warn you :p :D :p

[dougjones31]

I am sure you know what you are doing. The type of thread affects the strength greatly as does bolt hardness. With the right choice of screw, thread pitch and size....you can make yourself happy with the numbers. I did.

[kotengu]

What kind did you use? Even with a grade 8 or better 1/4-28 set-screw I could not get something I would settle for in the fatigue-life calculations.

In the plain yield strength comparisons it was fine, but using the worst case scenario (of the full 62 ksi on the bolt face) of multiple case head separations it showed a definite finite life in the fatigue calculations. IOW - the stresses were high enough that it would weaken the steel over time to apoint of failure, but not high enough to cause an immediate failure. Accurately predicting how many cycles that will be is difficult given all of the uncertainties, which is why I say go carefully. Assuming you never have a case head separation it should be fine indefinitely, but if somebody out there is naive enough to think I know what I'm talking about and does this mod, then ends up with a set-screw implanted in their forehead because they had several overpressure rounds or case head separations I would feel really badly, and partly responsible if I didn't put some kind of warning out there.

After all - nothing is foolproof to a sufficiently talented fool ;)

If anyone's curious I'd be happy to share my work, and the FEA images of the carrier (with and without the hole), but it's a little dry (not to mention would take too much space) to post for general viewing......

[Court in Fl]

I wonder if you could tighten things up by getting a locking shoulder that is a few thousands taller than the one you have.
I measured about 10 locking shoulders and most where .265 in this area,
but I found 2 or 3 that where around .267 to .268.

This would lift the bolt closer to the carrier.

Just an idea.

Court in Fl.

[Potshot]

I was thinking of hard-chroming the carrier. This should tighten up the bolt to carrier to locking shoulder by 2x the plating thickness. 1x in lowering the carrier itself in the reciever, and 1x in lowering the bolt relative to the carrier. Wear resistance would obviously be improved also.

I don't like the setscrew deal as the contact stresses at the screw head (didn't look @ thread root) are probably too high for the material (especially considering the material of ordinary setscrews). 4140 really ain't all that special either.

[Randyqnq]

I was thinking of hard-chroming the carrier. This should tighten up the bolt to carrier to locking shoulder by 2x the plating thickness. 1x in lowering the carrier itself in the reciever, and 1x in lowering the bolt relative to the carrier. Wear resistance would obviously be improved also.
Now I like that but where can I get the parts Chromed??

[cutlass1972]

I believe that this and its parent thread are really intriguing. I just have one question though. How would you go about actually measuring "slop" between the bolt and carrier? Would you just add some material to the back of the bolt and mill / file a few thousandths at a time until everything fit and locked up reliably?

[dougjones31]

Originally posted by kotengu
What kind did you use? Even with a grade 8 or better 1/4-28 set-screw I could not get something I would settle for in the fatigue-life calculations.

Try something a little larger with courser threads. I cannot remember exactly what I used and I do not have it handy to measure.

[kotengu]

That's the problem - 1/4" OD is max because of where it sits on the carrier. The 1/4" hole is about the largest you want to put there without completely removing the top rib. I'd even prefer to go with a finer thread for the screw, as there just isn't that much steel in there to give a good amount of thread "grip". You need a bare MINIMUM of three full threads of engagement, and between the small length of the screw and the existing hole for the rat-tail spring and plunger I think a 28 tpi is about the coarsest I'd go. In addition, the finer thread gives you easier control over the adjustment.

Also - the coarser thread (1/4-20 for example) gives you a smaller useful cross-sectional area than the finer thread, effectively increasing the stress placed upon the bolt. It's true that very generally speaking a coarse thread is stronger than a fine thread, but in this case I don't think it's the right solution.

[chrsdwns]

This is very intriguing and nice work. It seems to me that Doug Jones has the best idea of mixing and matching parts to get the best fit up, but how is that quantity measured? The size of the locking shoulder would be a factor in the equation as well since a smaller shoulder would tend to drop the bolt engagement point lower and increase the slop in the bolt/ bolt carrier fit up (ie the fit up is a function of relative location in space of the locking shoulder and the bolt and carrier). Checking the slop in the assembly after installing the locking shoulder would seem to be the best way.

I think that using a spray metal coating would be a good way to do the build up because it would not affect the heat treat on the base metal of the bolt like welding would. Spot plating would also work but the parts should be baked afterwards to prevent hydrogen embrittlement.

It also brings up the issue of why does this have an effect on stringing. Is it because of mechanical issues or is it because of altering the timing of the extraction cycle which would impact the pressure vs time curve in the barrel. If the slop is causing the bolt to begin extraction early in a random fashion could cause a sudden sharp drop in chamber pressure. Some velocity data would be nice to see if there is change the velocity of the bullet significantly from shot to shot, which would obviously have an impact on stringing.

[dougjones31]

If you play around with bolt/carrier combos then you can tell if they are a good match by measuring from the bottom of the receiver rails down to the bottom of the receiver and then matching that with the measurements from the carrier rails down to the bolt.

I get as close as possible then measure and install the LS. If there is slop after that then I just swap parts until it is gone then remeasure the headspace and swap the LS if I have to(uncommon). Court had a good point about the thickness of the LS. But I usually only see .001" difference in thicknesses. If I have to swap LS's, I always make sure that the new LS is the same thickness as the old one so that I do not introduce anymore slop.


This is easier than welding or plating. I just do not think any spray metal coating will hold up good enough to work.


I do not know what the exact cause of the stringing is...but removing carrier/bolt slop fixes it....and that is good enough for me.

[dougjones31]

Originally posted by kotengu
That's the problem - 1/4" OD is max because of where it sits on the carrier. The 1/4" hole is about the largest you want to put there without completely removing the top rib. I'd even prefer to go with a finer thread for the screw, as there just isn't that much steel in there to give a good amount of thread "grip". You need a bare MINIMUM of three full threads of engagement, and between the small length of the screw and the existing hole for the rat-tail spring and plunger I think a 28 tpi is about the coarsest I'd go. In addition, the finer thread gives you easier control over the adjustment.

Also - the coarser thread (1/4-20 for example) gives you a smaller useful cross-sectional area than the finer thread, effectively increasing the stress placed upon the bolt. It's true that very generally speaking a coarse thread is stronger than a fine thread, but in this case I don't think it's the right solution.


Did you think about drilling the hole at an angle from the front of the gun so that the forces do not directly affect the thread strength?;) If it is done this way then if the screw does fail it will shoot out away from your face.

[kotengu]

Did you think about drilling the hole at an angle from the front of the gun


Hmmm - interesting idea, but I think it would be difficult to do, between machining a flat to start your drilling to shaping the base of the screw to make good contact no matter how it is oriented. Good thinking outside the box (or the carrier), though........

[tac-40]

dougjones31
Your problem must have been big, but I am sure that more choices of carriers and bolt would have allowed you to find a perfect pair.

d31 - My stringing was in the neighborhood of 6 to 10 inches. The rifle would put a pair of holes next to each other, then the next pair would be about 4 inches higher, then a repeat. The final group would range about 4-6 inches diameter about 10 inches higher than the first two shot group. It no longer strings vertically but still has a four inch group size. I think that this can be tightened up a lot if I tinker with it some. Something to keep me busy.

I believe that this and its parent thread are really intriguing. I just have one question though. How would you go about actually measuring "slop" between the bolt and carrier? Would you just add some material to the back of the bolt and mill / file a few thousandths at a time until everything fit and locked up reliably?

I measured the bolt, BC, and receiver similar to what dougjones31 stated above. After TIG welding a bead on my bolt, I used a dremel and flat file to get the contour I desired. After that it was a couple of strokes with the file, measure the amount removed, and then test the fit-up. When I got where I wanted, I then had to file some more until I got the bolt to close on a dummy round and the headspace was verified to be in spec. The whole thing took about 4 hours to measure, weld, and fit-up the modified bolt. Took it out the same day to test fire it.

[Court in Fl]

Is there any way to put the screw in the bolt ?
Then if it blew out it would be caught by the carrier.

Court in FL.

[dougjones31]

I though about that, but you only need a few thousandths to fix the slop. Putting the screw in the bolt has 1 big problem....the firing pin. You would have to put the screw in from the top of the bolt and then grind it down to almost nothing. That would make it a pain to replace. It would also be a PITA to get it to lock into place. Besides, drilling the bolt is sure to weaken it....and that would not be good.

[patrick20091]

Originally posted by tac-40
As of now, I have identified a couple of other things I want to try in order to wring out the best accuracy the rifle can give. I am happy with the performance now, but feel it can be better. So begins a long term project on improving the accuracy of this particular rifle.

Been following this thread. Amazing that a mere .004ths cured your problem. I hope I'm so lucky. I have the same problem with mine, except my groups string even worse. I'm glad you went ahead with the project. This gives me confidence since there had been objections based on the welding heat affecting the bolt, but the weld only takes a second to apply so I'm gambling that there's really not enough heating to significantly affect the overall strength of the bolt in that area.

With a dummy round using fresh, resized brass in the chamber I had .015ths of vertical slop at the rear of the bolt. Side to side is not too bad. Following your example, I welded a pad on the bolt using a 7014 rod and worked that down using a file and sandpaper. This reduced the slop down to about .004ths. I'll see tomorrow if it helps.

[jerrymrc]

Originally posted by patrick20091


Been following this thread. Amazing that a mere .004ths cured your problem. I hope I'm so lucky. I have the same problem with mine, except my groups string even worse. I'm glad you went ahead with the project. This gives me confidence since there had been objections based on the welding heat affecting the bolt, but the weld only takes a second to apply so I'm gambling that there's really not enough heating to significantly affect the overall strength of the bolt in that area.

With a dummy round using fresh, resized brass in the chamber I had .015ths of vertical slop at the rear of the bolt. Side to side is not too bad. Following your example, I welded a pad on the bolt using a 7014 rod and worked that down using a file and sandpaper. This reduced the slop down to about .004ths. I'll see tomorrow if it helps.

Talk about bringing up one from the dead.:beer: And I remember this one well.

[patrick20091]

Originally posted by jerrymrc


Talk about bringing up one from the dead.:beer: And I remember this one well.

Ha! I never looked at the date of the last post. As it turned out I lost the damned firing pin spring anyway, so I'll have to wait for another one to arrive. Oh well, the thread is only five years old, so I guess I can wait a couple weeks.

I wonder if Tac-40 ever got his gun shooting right?

[tac-40]

It isn't a target gun but it does hold a nice 2-3 moa group at 100 yds with no stringing. She has about 10k rounds through her now. I have used it in competition for 3 gun and tactical matches. I haven't tweaked it any more because it is 100% reliable and eats everything I put in her.

And yes, this is an old thread. This rifle I worked on was my second build: 16 1/2 inch G1 specifically put together for tactical matches. I call her my Kombat Karbine. I shortened the barrel because the last few inches of the kit's barrel was oblong due to some vigorous and unhealthy cleaning practices. Just kept cutting her down until I got O instead of 0.

[patrick20091]

Originally posted by tac-40
It isn't a target gun but it does hold a nice 2-3 moa group at 100 yds with no stringing. She has about 10k rounds through her now. I have used it in competition for 3 gun and tactical matches. I haven't tweaked it any more because it is 100% reliable and eats everything I put in her.

I'm glad you checked in with your results. Today I tried it out at the range and tightening-up the lock-up definitely made a big difference. (I found that firing pin spring I thought I lost.)

Hand feeding rounds gave me 1 inch groups at 50 yards. Out of the mag, I get 2.5 inch groups, still with some stinging. I also compared slow-fire to rapid fire and the groups stayed about the same, so I think I can rule out barrel heating as the culprit. The first round from the mag gave a perfect hit and the rest went higher. I'm thinking now that finding the right load will bring me into the 3-4 MOA range or maybe better. I'd like to get down to 2 MOA, but I can at least live with 4. Anything is better than the 12+ MOA I was getting before.