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View Full Version : BHO locking on front of carrier instead of bolt . . .


ratas calientes
January 09, 2017, 01:23
Folks -

So I was out dialing in my adjustable gas block on a 300 Blackout SBR. I would load the magazine with one subsonic round at a time, shoot it, and check to see if the BCG (Bolt Carrier Group) locked back on the BHO (Bolt Hold Open). If not, I would increase the port slightly (drill to the next size numbered drill bit) and repeat. (Gas block is MicroMOA Guvnah with the port plate.) I noticed that the BHO was too often locking back on the bottom front of the Bolt Carrier . . . . and I needed to keep increasing the gas even a few more drill sizes to get it to lock back on the front of the Bolt itself. Is this normal in any way? It concerns me because I reached a gas port size of 0.113 before I stopped. According to my research, I expected to reach a functioning unsuppressed subsonic gas port size of around 0.104.

This evening I checked this SBR, and my other 300 BLK, and I was also able to duplicate it without firing be pushing on the BHO as I pulled back slowly on the Charging Handle. Lower parts kits on these two rifles is CMMG. I could not duplicate this on one of my much older rifles made from Bushmaster parts. This older rifle would only lock back with the BHO in front of the Bolt itself.

Initial inspection of the fronts of the Bolt Carriers looked the same, along with the BHOs. I need to get them out again when I have more time and compare them side-by-side in better light.

Thoughts?

NFADLR
January 09, 2017, 01:46
Are you using the same bolt and carrier on both setups ?

Weight of carrier could vary from M16 carrier to cut off bottom on an ar15 carrier.

Buffer weight ?

fmj_shooter
January 09, 2017, 01:57
Folks -

So I was out dialing in my adjustable gas block on a 300 Blackout SBR. I would load the magazine with one subsonic round at a time, shoot it, and check to see if the BCG (Bolt Carrier Group) locked back on the BHO (Bolt Hold Open). If not, I would increase the port slightly (drill to the next size numbered drill bit) and repeat. (Gas block is MicroMOA Guvnah with the port plate.) I noticed that the BHO was too often locking back on the bottom front of the Bolt Carrier . . . . and I needed to keep increasing the gas even a few more drill sizes to get it to lock back on the front of the Bolt itself. Is this normal in any way? It concerns me because I reached a gas port size of 0.113 before I stopped. According to my research, I expected to reach a functioning unsuppressed subsonic gas port size of around 0.104.

This evening I checked this SBR, and my other 300 BLK, and I was also able to duplicate it without firing be pushing on the BHO as I pulled back slowly on the Charging Handle. Lower parts kits on these two rifles is CMMG. I could not duplicate this on one of my much older rifles made from Bushmaster parts. This older rifle would only lock back with the BHO in front of the Bolt itself.

Initial inspection of the fronts of the Bolt Carriers looked the same, along with the BHOs. I need to get them out again when I have more time and compare them side-by-side in better light.

Thoughts?

What's the size is the port on the barrel. Adjusting port size on block will not change anything if the barrel port size is too small.

Eta Are you drilling port plates or barrel?
Eta again. I've build several blackouts 16" with carbine gas, that needed .115 port dia to function subs correctly. (Just checked my ref log)

gunplumber
January 09, 2017, 07:49
Same thing happened on a customer's Noveski 300 BO as we were tweaking the gas for shorter length. That it needed more gas wasn't in question - that the BHO could latch on the carrier was curious.

Sagerider
January 09, 2017, 08:34
JP Enterprises from MidWay and the various weight spring package would solve the problem. I had the same problem and put in the next lighter spring, no more problem.

http://www.midwayusa.com/product/767861/jp-enterprises-silent-captured-recoil-buffer-spring-assembly-gen-2-ar-15

http://www.midwayusa.com/product/487681/jp-enterprises-alternate-spring-pack-for-ar-15-silent-captured-spring-recoil-buffer

jhend170
January 09, 2017, 11:24
Blackouts with subs is all about mass. It doesn't like a lot of it. I actually run an Adams Arms piston setup on mine, but with the lightened carrier. Works great and serves as my truck gun and impromptu pig slayer with a RDS and magnifier.

It and 3 full p-mags makes for a lot of "wrong victim mother $#@%^!!!!"

Hell it's even a pistol so I COULD carry it concealed, were I that much of a masochist.

ratas calientes
January 09, 2017, 23:28
SBR build details:

RRA lower
PSA upper
CMMG lower parts kit
8" CMMG barrel
MicroMOA Govnah gas block w/ three gas port settings
PSA BCG, full-auto bolt carrier
Standard carbine buffer
H2 buffer


Answers to questions:
First setup was SBR. I was able to get the BHO to hold back the BCG from the front of the Bolt Carrier. I was able to repeat this on a second carbine that also uses CMMG lower parts kit and another PSA BCG.

I drilled out the port in the barrel to 0.116 before I installed the gas block.
*** I should checl the alignment of the barrel gas port with the gas block ***
I was drilling out the port plate.

I probably can't examine the carriers and BHOs in the various rifles until this weekend. I'll get back to you.

fmj_shooter
January 10, 2017, 02:13
"H2 Buffer"
You can take weight out or put standard carbine buffer in.
H2 is 1.7 oz heavier than standard carbine buffer.

gunplumber
January 10, 2017, 08:45
This is only anecdotal, as the rifle in question had one of those side-folding stocks, so the buffer was different.

I finally got to take my 300 Blackout pistol out for a spin after you
worked on it. It is an awesome pistol!!

I shot supersonic with and with out a can and it worked perfectly.

I shot subsonic with a can and it worked fine and even locked back on
the bolt properly after the last round. It was catching on the bolt
carrier when you shot it.

When I shot subsonic without a can it would eject the spent casing but
not pick up a new round.

It cycled subs with the original Flaming pig, 10.5" barrel and no
folder. We did three things to the pistol; shortened the barrel to 9",
put a AAC flash hider and the Laws Tactical Folder. The Folder has a
piece that is attached to the bolt carrier that weighs approximately 3
oz. I took 2 weights out of the carbine buffer and put a wood dowel in
there. Thinking to normalize the weight of the bolt carrier and buffer
with the extra 3 oz. I shot it today and it did cycle a few rounds,
maybe worked 20% of the time and got a mostly no feeds and a few
incomplete feeds. It did eject each casing.

I would really like to be able to cycle subsonic with a can and
without. Even if it cycles a bit harsher with the supersonic. I don't
want to do too much to the buffer and spring because I want to be able
to put the 5.56 barrel on and shoot it.

Do you have any ideas? I was thinking of having you make the gas port
a bit larger.

Thanks Don

I'm thinking the gas port is the way to go - there are two challenges to this.

First - the gas block is on extremely tight, which is more of an annoyance than a problem. I used an oak block to drive it off last time. May end up scratching the parkerizing.

The second is, I don't know how much bigger to make the gas port.

Extrapolating from other guns, I'm thinking 13-14% larger will compensate for the shorter barrel. On the FAL, shortening the barrel from 21 to 18 means opening the gas port from .100 to .116. That's about 14% shorter barrel and a 14% larger gas port.

If we shortened your barrel from 11" to 9", that's 18%. So an 18% larger port? I don't know if it works that way, so I'd prefer to be conservative, even if we have to do it twice - better than making it too big.

And some notes I pulled from others' research.

9.5″ WITH PISTOL GAS SYSTEM (THIS BARREL CAME WITH A .086″ PORT FROM FACTORY)
MINIMUM PORT SIZE UNSUPPRESSED WITH SUBSONIC AMMO
Remington 220 Gr .093″
Hornady 208 Gr .093″
175 Gr Reload .093″
MINIMUM PORT SIZE SUPPRESSED WITH SUBSONIC AMMO
Remington 220 Gr .093″
Hornady 208 Gr .093″
175 Gr Reload .086″
MINIMUM PORT SIZE UNSUPPRESSED WITH SUPERSONIC AMMO
Remington 115 Gr .073″
MINIMUM PORT SIZE SUPPRESSED WITH SUPERSONIC AMMO
Remington 115 Gr .067″

This barrel length seems to be the sweet spot for 300BLK. We didn’t test a 9″ but it is probably very close to this 9.5″.
For someone that is going to be shooting subsonic and supersonic unsuppressed then a .093″ port would work fine and forget about an adjustable block unless you wanted to kill the gas.
When going to supersonic suppressed the spread in port sizes gets bigger and I think an adjustable gas block works well for this particular barrel.
Proposed port dimensions to run all 4 configurations: (.093″/.073″)(Subsonic/Supersonic)
Again this would be for someone that would want to run their 9.5″ in all 4 configurations mentioned.
A footnote about this configuration:
As noted this barrel came with .086″ port. Turns out that the blank manufacturer had a looser bore on a later batch of barrels so they had to go to a larger port size to compensate which is currently .093″.
Could