Does bolt carrier affect headspace?

[Bwana John]

I bought a sand-cut carrier and bolt at the gunshow yesterday, can I switch out only the carrier without affecting headspace?

[Texas Patriot]

Maybe, but it always better to recheck headspace when you change any component of bolt or carrier.

Have it checked, you'll sleep better knowing its right.

[Pa. Patriot]

The carrier absolutely affects HS.
The contact of the bolt to the carrier is unique any any pair.

Changing the carrier, in my experience, changes the headspace more than changing the bolt, and it will change it nearly every time by as much as several .001

[MisterJG]

Yes, Don't ask me how. But it does.

[Temp]

Someone's going to have to explain to me how the carrier affects the headspace. The headspace is determined by the distance between the rear shelf and the face of the bolt, and it's relationship to the thickness of the locking shoulder. The depth of the chamber also obviously plays a part in the overall scheme of things, but when the bolt drops into battery, the carriers job is finished until the next cycle.

Tell me how the carrier affects the distance between the rear shelf of the bolt and the boltface,... and it's relationship to the locking shoulder.

[EMDII]

Yes, Don't ask me how. But it does.

Carriers do NOT affect headspace. HS is set by the bolt only. The BC is a lock/unlock/mass system. Only if the carrier is out-of-spec could it possibly (and improbably) affect HS. More likely the beast wouldn't cycle.

See THIS thread by the way in R&D and scroll down to Interchangeability. Select it: you'll see several B/BC threads in the results.

[dougjones31]

Radio did a test one time where he swapped all types of bolts and carriers and only came up with a change of .001" at worst.

I have swapped several bolts and carriers and got no noticible change in headspace. It is best to double check with guages, but I think you will have no problem.

Although..I could see where an out of spec carrier would cause problems. If the carrier holds the bolt at a slight angle off axis of the bore...then the locking surface would be pushed back because of the off axis bolt. I would think that this sort of thing would always cause the headspace to be tighter with the out of spec bolt carrier. I have never encountered this....this is just a guess as to how the carrier could affect the headspace. I could just be crazy as hell!

[dwkennedy]

I tested this empirically. I checked headspace with pin gages, swapping several bolts and carriers. I believe I tried something like 6 carriers with the same bolt, closed on the go-gauge with the pin each time (and wouldn't close on the next larger pin.) So I vote for the carrier-doesn't-affect-headspace crowd. I still check it anyway with the gauges when I swap something. One extra piece of insurance never hurts.

And, as a note, all of my bolts ended up a couple thou either way, I could probably get away with swapping any of them without checking, provided the correct locking shoulder was used. I still check them anyway, have to justify buying that high-dollar DSA bolt disassembly tool

[MisterJG]

I'm not trying to confuse anyone with my .02. But when I was headspacing my Imbel on Imbel build my math indicated I needed a .249" L.S. + .001" for set back (.250") I got ahold of a O.E.M. that mic'ed at .251". Once I installed the L.S. I know I was at a 1.630" a tad tight for surplus ammo. I went out to the range and shot a box of Winchester .308 silvertip without a problem. Once I started to shoot some surplus Calvim and Radway. I started to experiance some FTE with the Radway ammo. Once I got home I stripped & cleaned the rifle down and checked the headspace and the .001" setback did occur, which now had my headspace at 1.631" (my goal was 1.632"). So using my forster go gage & some tape (1.632" go gage), I went about lapping the bolt to the L.S. When I first tried lapping with the carrier in place the bolt would not come close to completely closing. But if I removed the carrier the bolt would drop right in closing the chamber and locking itself on the locking shoulder.

So this is how I came to the conclusion that the carrier does indeed effect headspacing. I don't know the acting forces, or how the engagement tabs all tie in. All I do know is Doug and Radio have been doing this alot longer then I have and they know their sh#t. I was just stating the facts as I know them. As a safety measure I would always double check.

[Pa. Patriot]

Originally posted by EMDII



Carriers do NOT affect headspace.

The thread you posted even confirms that the carrier can affect HS.

I've swapped many a carrier around and they can and do affect HS.
Especially Imbel carriers.

I really don't have time on the how or why right now but suffice to say that my measurements conclude that the carrier absolutely can affect HS.

[dougjones31]

Perplexing. Some have had problems and some of us have not seen much deviation at all. Maybe we are doing something differently. Sitting here viualizing the way the action works, I can only see the off axis possibility. I put 3 different Imbel carriers and 1 G-1 carrier into an Imbel yesterday. The headspace was 1.632 and each swap allowed the bolt to close on a 1.632 guage and not close on a 1.634 guage. So, I had no problems.

As the carrier goes forward, the carrier lets the bolt ride freely until it hits the next round in the mag and then the bolt is pushed bacwards and downward in the carrier. When the bolt face wedges against the newly chambered round the carrier still moves forward. This forward motion, combined with the cut in the carrier, forces the bolt downward. This downward force will make the bolt drop into place only if the headspace is large enough for the bolt to fit. Same bolt and same LS then the fit should be the same. I have never seen any significant change.

If you were to use just the bolt without the carrier, you could in fact move the rear of the bolt to either side and it would not lock. Now....if the bolt cutout in the carrier or the rails of the carrier were not square, then you would get this sort of problem...but it would always make the headspace tighter because of the binding. I have never seen a carrier that caused this.

Now.... the carrier will only let the rear of the bolt drop so far before the firing pin stops it, so I guess some carriers might not let rear of the bolt drop enough to fully lock into place? This would cause the headspace to look looser, but in fact you would have a dangerous situation where the bolt is not fully seating. I guess you would have to enlarge the hole in the carrier where the firing pin goes through, to allow the bolt to drop down further. HMMMM...I have never seen this either, but if someone was not paying close attention...I guess this could happen. This would be simple to fix with a round file.

This is all purely speculation. I was just trying to visualize how the carrier and bolt interact. The bolt is the same length and the headspace is the same length....unless the carrier is binding the bolt...then the the swap should not effect anything.

My head hurts now!

[ByronF]

I suspect some of us could check headspace several times with the SAME bolt and carrier, and STILL get slightly different results with each trial. So I'm not shocked that changing carriers might appear to change HS by 0.001 or so. But that doesn't mean that changing carriers causes a statistically significant variation in HS. The deviation from changing carriers is likely on par with the deviation from taking several measurements with the same bolt and carrier (according to the experimentor's proficiency with taking headspace readings). Like Doug said, if each carrier allows the bolt to fully seat down against the shoulder there is no way the carrier can affect headspace.

Byron

[Pa. Patriot]

Originally posted by ByronF
I suspect some of us could check headspace several times with the SAME bolt and carrier, and STILL get slightly different results with each trial. So I'm not shocked that changing carriers might appear to change HS by 0.001 or so. But that doesn't mean that changing carriers causes a statistically significant variation in HS. The deviation from changing carriers is likely on par with the deviation from taking several measurements with the same bolt and carrier (according to the experimentor's proficiency with taking headspace readings). Like Doug said, if each carrier allows the bolt to fully seat down against the shoulder there is no way the carrier can affect headspace.

Byron

This is simply not the case in my testings. In my tesing I never had ANY variation between tests with same bolt/carrier combinations.
Have had several .001 (like .003+) differences after swapping carriers.

All bolts I have tested (over a dozen) WITHOUT a carrier come up within .001 of each other. It is when the carrier is present that the variation changes to beyond .001 - in some instances.
Original bolt/carrier combos always yielded the same measurements. I repeat, I had no "deviation from taking several measurements with the same bolt and carrier".

[Temp]

I'd never thought about the issue until I tried to headspace a rifle that I had rebarreled. I had previously headspaced the rifle with another barrel and the locking shoulder was left in place in the receiver. The chamber of the new barrel was cut ever so much shallower than the previous barrel and the bolt would offer just a bit of reluctance to go into battery with a no-go guage. I had 5 bolts and 5 carriers to use and I tried various bolt/carrier combinations with no success.

I *did* find that the Imbel bolts in my possesion guaged a bit longer than my FN bolts,.. about .002". I'd try an Imbel bolt with all 5 carriers, and the bolts wouldn't go into battery. Then I'd try an FN bolt with all 5 carriers and the bolt would go into battery with slight thumb pressure. At no time did a change in carriers affect the headspace. It was at that point that I put 2 and 2 together and realized that the carriers play no part in the datum between locking shoulder and the face of the no-go guage. Only the bolt itself spans the distance between those two points. The carrier is just along for the ride.

I pecked the locking shoulder out, stoned .0015" off of it and lived happily ever after.

[Radio]

I also have not had differing readings when making headspace checks with the SAME bolt and carrier. In fact, my two watchwords when headspacing are REPEATABILITY and CONSISTENTCY.

The test I performed here over two years ago was between SEVEN bolts and SEVEN carriers and headspace measurements between ALL combinations. Then I did the entire test series a SECOND time to check my readings (they were the same). Usually, swapping the bolts affected headspace more than swapping carriers, but the maximum difference never exceeded .002" in any of my tests. Carriers USUALLY didn't change things more than .001", if at all.

Mostly, you're getting the same advice: "Probably Not" and "Check It Anyway".

--Radio

[dougjones31]

Radio...

Were you snoozing? I thought you were never gonna comment. Ooops I thought your max varience was .001". Oh well...I guess I was wrong this time! They say your memory gets bad as you get older....I guess I am getting older.

I just do not know how PA Patriot is getting the big variation. Must be one of those PA things.....kinda like the twilight zone up there! Just kidding Pa Patriot.

[faner50]

Think of it this way. The carrier carries the bolt from battery to ejection and back to battery. When the bolt is in battery the LS holds it in place in proxcimity to the case shoulder which is where HS is measured. The only thing the carrier could effect is going to battery or not . It would not , could not and should not have any effect on HS. Just my $.02

[Radio]

If that were so, we'd all be checking our headspace with just the bolt... WITHOUT the carrier. But we wouldn't be getting true readings. Just look inside your rifle and see why.

That bolt is NOT independent. There is a specific relationship between it and certain camming actions taking place inside the carrier, as the two parts move together in the receiver... not only of the bolt down against the locking shoulder, but of the bolt inside the rear of the carrier:
* at the top rear of the bolt directly above the firing pin, against the inside top rear of the carrier;
* at the camming ramps on each side of the rear of the bolt, against corresponding bosses in the inner sides of the carrier;
* at the end of the firing pin, against the inside top rear of the carrier, just above the firing pin hole beneath the rattail.
All this metal-to-metal contact is going on as the bolt is being forced down and out of the carrier (thus down against the locking shoulder). What's more, the bolt is being held in a specific attitude within the receiver as it moves... by the carrier. Yes, Virginia, the carrier is VITAL to the bolt's movements.

In a recent thread I offered some strongly-flavored opinions about not removing the firing pin when making headspace measurements. While I will admit (here, at least) that my viewpoint on this matter may possibly be a bit fanatical, it IS a fact that the firing pin is banging against the inside of the carrier during the bolt camming action... just look at the wear patterns.

These components are all part of a system designed to work together. Will your headspace readings be thrown off if made without the firing pin in place? Maybe, possibly, perhaps not. But the firing pin DOES have SOME effect on the bolt's freedom of movement. Will your headspace readings be thrown off if made without the carrier in place? You bet your bippy. The carrier is an INTEGRAL part of the bolt's camming down into lock.

In closing, facts from opinions:
* The carrier is WHY the bolt cams down against the locking shoulder.
* "The contact of the bolt to the carrier is unique in any pair."
* I and others have MEASURED headspace changes when using different carriers and the same bolt.
* Sometimes there are NO headspace changes when using different carriers and the same bolt.

I'm back where I started. "Will the headspace change if I swap carriers?" Maybe or maybe not, but probably not much. Check the headspace after the swap anyway.

--Radio

[Jambo]

(I know that this thread is dated, but, what the hell.)

I have to agree with dougjones31, Temp, faner50, and ByronF; doug esspecially:

Originally posted by dougjones31
If you were to use just the bolt without the carrier, you could in fact move the rear of the bolt to either side and it would not lock. Now....if the bolt cutout in the carrier or the rails of the carrier were not square, then you would get this sort of problem."
"the carrier will only let the rear of the bolt drop so far before the firing pin stops it, so I guess some carriers might not let rear of the bolt drop enough to fully lock into place? This would cause the headspace to look looser, but in fact you would have a dangerous situation where the bolt is not fully seating. I guess you would have to enlarge the hole in the carrier where the firing pin goes through, to allow the bolt to drop down further."
"The bolt is the same length and the headspace is the same length....unless the carrier is binding the bolt...then the the swap should not effect anything.

Originally posted by Temp
Only the bolt itself spans the distance between those two points. The carrier is just along for the ride.

Originally posted by faner50
The only thing the carrier could effect is going to battery or not . It would not , could not and should not have any effect on HS.

ByronF summed it up beautifully:

if each carrier allows the bolt to fully seat down against the shoulder there is no way the carrier can affect headspace.

Hey, I got an idea: why don't all of you guys that conducted the little tests using the same bolt, or a number of bolts, with different carriers conduct the same experiment twice: once with the firing pin in place and again with the firing pin taken out. Then we'll see what the hell the problem is.

[EMDII]

I'm no 'builder, so I won't argue. Rather, I'll say that a properly made BC should only perform the lock/unlock function. HS is a function of the Bolt/LS relationship to the installed barrel/chamber. IF a BC is making .003" change, I'd have to say that mismatch is because of different versions of the B/BC that MAY not allow the Bolt to go fully DOWN (yes down) on the LS.

JMO-

There were at least three versions of the bolt, one a single cam and the others double cammed, and at least five versions of the BC, w/ two different internal cuts. So, a good practice is to verify that you have the same BC, AND to inspect HS after changes.

[Radio]

Ted has spoken. Bottom line: ALWAYS re-check headspace when changing something!!

I understand the perplexity of those who claim the carrier should not affect headspace. Theoretically, the bolt will go as far forward as the round will allow, and drop with a certain clearance in front of the locking shoulder. Period.

I and others who have actually MEASURED know this is strangely not entirely true. I discovered there was OFTEN NO CHANGE, but SOMETIMES there was a SLIGHT change. I can only attribute this to:
* the unique relationship that any two parts will have due to wear and machining tolerances, or:
* various versions of these parts in use as detailed by Ted.

Here are my test results again, from March 14, 2001. I took painstakingly careful measurements, then performed the entire series a second time to double-check my readings. They were the same. Five of my seven bolt/carriers were from numbers-matching kits.

I’ve broken this down into two sections for your convenience, first by bolt (“1” thru “7”) and then by carrier (“A” thru “G”). The bolts:

• Bolt-1 is StG/Austrian, “Excellent”, matches Carrier-A.
  A-259 B-259 C-259 D-260 E-259 F-259 G-259
• Bolt-2 is StG/Belgian, “Very Good”, matches Carrier-B.
  A-260 B-260 C-260 D-261 E-260 F-260 G-260
• Bolt-3 is South African, “Good”, matches Carrier-C.
  A-259 B-259 C-259 D-259 E-259 F-259 G-259
• Bolt-4 is South African, “Good”, matches Carrier-D.
  A-260 B-261 C-261 D-261 E-260 F-260 G-260
• Bolt-5 is South African, “Good”, matches Carrier-E.
  A-260 B-260 C-261 D-261 E-260 F-260 G-261
• Bolt-6 is Belgian, “Excellent/New” (entire FP is black except for very tip).
  A-261 B-261 C-261 D-261 E-260 F-261 G-261
• Bolt-7 is South African, “Good”.
  A-260 B-260 C-260 D-260 E-260 F-260 G-260

Note: Bolts 3/7 didn’t care which carrier they were in, same reading all seven. Bolts 1/2/6 varied in only one of seven carriers by .001. Now the carriers:

• Carrier-A is StG/Austrian, “Excellent”, matches Bolt-1.
  1-259 2-260 3-259 4-260 5-260 6-261 7-260
• Carrier-B is StG/Belgian, “Very Good”, matches Bolt-2.
  1-259 2-260 3-259 4-261 5-260 6-261 7-260
• Carrier-C is South African, “Good”, matches Bolt-3.
  1-259 2-260 3-259 4-261 5-261 6-261 7-260
• Carrier-D is South African, “Good”, matches Bolt-4,
  has high wear polish along bottom, the hammer from this kit also significantly burnished.
  1-260 2-261 3-259 4-261 5-261 6-261 7-260
• Carrier-E is South African, “Good”, matches Bolt-5.
  1-259 2-260 3-259 4-260 5-260 6-260 7-260
• Carrier-F is Belgian, “Excellent/New”, virtually no wear marks.
  1-259 2-260 3-259 4-260 5-260 6-261 7-260
• Carrier-G is South African, “Good”.
  1-259 2-260 3-259 4-260 5-261 6-261 7-260

Note: Carriers A/B/F/G varied by .002, majority of readings in center. Carrier E varied by only .001, majority of readings at high end.

Conclusions: Exchanging bolts in particular carriers varied headspace by only .002, from .259 to .261, with the majority of the seven measuring .260. Exchanging carriers with particular bolts affected headspace even less, by only .001, and often not at all.

While this test included only seven of each component, they were of varied manufacture and condition. Obviously it would be better to have a larger sample, say 100 bolts/carriers, for more conclusive results, but based on the information gathered here it appears that, so long as you headspace towards the middle, swapping in spare bolts/carriers may be entirely practical. Of course, “your mileage may vary” and ALWAYS check headspace after swapping.

--Radio