Would like help diagnosing problem with 300 BLK AR

toddrod · Apr 20, 2015

So I just loaded up my 1 st rounds for my 300 AR upper and it is not cycling the bolt and would like some help figuring out the problem

Equipment

Brand new New Frontier carbine polymer complete AR lower
Brand new Hardened Arms 300 BLK complete AR upper with 16 inch barrel with pistol length gas system
Gas port 0.093
Troy Claymore muzzle brake
Load = 125g Sierra prohunter with 18.3g H110 as per Seirra load book. (max load per Seirra)
20 rounds run through it so far

So could it be that everything is new and needs a break in?
Is the buffer, or spring to heavy.

Would appreciate your thoughts on this.

noob · Apr 20, 2015

Doesn't cycle at all? or just doesn't reliably grab the next round?

Did you check you gas block and made sure it's aligned with the hole on the barrel?

Manually cycle rounds to see if it's really tight?

Sig220 · Apr 20, 2015

Are you running it wet? Sometimes with a mix of parts you have to lube it up and give it a chance to settle in.
Otherwise, you could have any number of problems from a bad gas block alignment to a binding buffer.

Does it cycle easy manually by hand, and if so will it lock back on a empty mag? I know a guy who bought a buffer tube off of egay. Couldn't figure out why it wouldn't feed from the mag....it turned out the buffer tube was too short the bolt was not coming back behind the mag to have a chance at stripping a round from the mag. Replaced the buffer tube and away it went!!

SpeedRacer · Apr 20, 2015

noob said:
Did you check you gas block and made sure it's aligned with the hole on the barrel?

This would be my first guess. But we need more info, is it not cycling at all, or just short stroking (ejecting spent shell but not feeding next round)?

Gator 45/70 · Apr 20, 2015

Jesus! That's damn near a 44 mag load, I'd cut back some.

Jack · Apr 20, 2015

Gator 45/70 said:
Jesus! That's damn near a 44 mag load, I'd cut back some.

It's for a 30 caliber rifle

Gator 45/70 · Apr 20, 2015

This is Hodgons load, He appears to be too hot.

Hodgdon

H110

.308"

2.060"

16.7

2,020

40,200 CUP

17.8

2,118

48,800 CUP

Jack · Apr 21, 2015

https://sierrabullets.files.wordpress.com/2013/07/300-aac-blackout.pdf

Gator 45/70 · Apr 21, 2015

Can't get into it, Blocked Zeon Corporation.

toddrod · Apr 21, 2015

Sig220 said:
Are you running it wet? Sometimes with a mix of parts you have to lube it up and give it a chance to settle in.
Otherwise, you could have any number of problems from a bad gas block alignment to a binding buffer.

Does it cycle easy manually by hand, and if so will it lock back on a empty mag? I know a guy who bought a buffer tube off of egay. Couldn't figure out why it wouldn't feed from the mag....it turned out the buffer tube was too short the bolt was not coming back behind the mag to have a chance at stripping a round from the mag. Replaced the buffer tube and away it went!!

Yes, it cycles pretty easy by hand and ejects the cases fine. It does lock the bolt back by hand when cycled manually. I have everything pretty wet with Rem Oil.

- - - Updated - - -

Jack said:
https://sierrabullets.files.wordpress.com/2013/07/300-aac-blackout.pdf

That is the information I am going by.

toddrod · Apr 21, 2015

It will be a couple of days before I am able to get some more rounds through it but what I have done is manually cycle the bolt approx 200+ times as suggested from someone to help break in the the action since this is all brand new stuff.

dougstump · Apr 21, 2015

toddrod,

I'm in the Shreveport/Bossier City area; if you're close I'd be happy to get together with you & try to sort out the problems. Try to find some 1680 powder; I’ve had good luck with it.

noob · Apr 21, 2015

If it cycles by hand easily but not when fired, i would take an air compressor and spray some air down the barrel and see if you feel it come through the gas tube.

dougstump · Apr 21, 2015

1. Is it extracting the fired case?
2. Is it ejecting the fired case, or stove pipe jamming?
3. If the fired case is ejecting, where is is landing & how far?
4. Is the bolt closing on an empty chamber?

dougstump · Apr 23, 2015

Considering the max load you are using, I suspect you are having “bolt bounce” from being over gassed. Instead of the bolt & buffer coming to a gentle stop, the buffer is slamming into the back of the receiver extension (buffer tube) and bouncing back forward. When this happens the magazine spring can’t push the next round up fast enough for the bolt to catch it (since it’s a new rifle I’m guessing you are using new magazines too). I’ve run into this issue on a few firearms.

I was having the same problem with my 6.5 Grendel (they are known for over gassing). Instead of installing an adjustable gas block I put in a Tubb carrier weight to slow the bolt down and the problem went away.

I would suggest loading a few rounds at the starting load (17.1 grains) or get a box of factory ammo and see how the rifle behaves. Always remember, the loads published in the manuals are within SAMMI pressure limits, but this doesn’t mean they will cycle your firearm. For example, IMR-4198 will cycle fine in a 5.56 AR-15 but not in an AR-180.

franch1se82 · May 2, 2015

the easiest solution might be to check if the gas block is aligned properly with the gas port on the barrel.

toddrod · May 24, 2015

Just revisiting this topic to update it. Today I finally made it to the range and tried handloads of 18.4g / 18.6 / 18.8g (max load) H110 with Hornady SST 125g bullets. all bullets fit my Sheridan ammo gauge (cut to min SAAMI specs). All rounds fired but the bolt did not cycle and the cases were hard to manually extract even though they showed no signs of being over pressure. So I then went bought a box of Barnes Vortex 110g black tips. Same thing, no bolt cycling with hard manual extraction. when I got home I pulled the BCG from my other AR and put it in the 300 BLK and all rounds functioned and extracted perfectly. So it is a bolt issue. They are of different manufacturers and both are the full auto versions. Looking at them I do not see any rough edges on the one that came with my 300 BLK upper or anything else not right. When I get a chance I will use my micrometer and take some measurements.

By the way those Hornady SST's are the cats meow. I was putting them right on top of each other with just a red dot at 40yds.

toddrod · May 24, 2015

I just measured the lugs on the the 2 bolts for my ARs.

Bolt for 300 BLK - lugs measured .1 to .12 in width
bolt for my other AR (the one that functioned) .098 in width on all lugs.

Also the length of the lugs are all different. They measure from .277 to .285. in length. None of them are the same. On my other bolt every lug is .278 in length

So can anyone suggest a good inexpensive bolt that I can order to replace this one?

toddrod · May 24, 2015

Just got word the BCG is under warranty and Hardened Arms will ship a new one.

toddrod · Jun 9, 2015

Final update. Install replacement BCG and this morning she ran like a scalded cat with 3 types of handloads I put through it.

Would like help diagnosing problem with 300 BLK AR

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