sierra 110gr, hp/ imr 4227 not locking back.

[devious169]

So, I built my first 300blk upper, mega mkm upper with wilson combat 16'' recon barrel, wc flashhider,wc adjustable gasblock and tube( running wide open) , wc semi bcg and charging handle. I function tested this with remington premier match 125 gr. 20 rds for function and 220 rds for break in with interval cleanings. All rounds were flawless, but 25-30 dollars a box! I decided to reload. Wanting to start with a light bullet and loving hollow points , I went with 110 gr sierra hp varminters. only powder with similar burn rate to h110 or w296 I could find was imr 4227. Starting with my brass,is lc 09 and was formed on rcbs ar dies, trimmed to 1.363 ( per imr test data). My starting load was 17.6,18.0,18.4,and 18.8 grains x 5 rounds per. Accuracy with the 18.4 grain was phenomenal and the 18.8 was damn good too! Only one catch, not a damn one would lock my bolt back! I ran a full auto 11.4 oz. and a semi auto 10.9 oz carriers and the buffer (i assume is a carbine) 2.8 oz. Any suggestions would be greatly appreciated.

 

[jblomenberg16]

What length is the gas system? Some of those faster burning powders work better with pistol length system vs. a carbine length system given the pressure curve "shape." If you have any heavier bullets to try, see what that does. I bet if you use some 147gr M80 pull downs it will run fine. Let me know if you need a few to try and I'll mail you some.

 

[devious169]

Its a carbine length gas system. Given that it will run 125 grain bullets, I wouldnt think that a 15 grain differential would cause this, although anything is possible. According to burn rate charts i have found imr 4227 burns similar to h110,w296 and lil gun.

 

[jblomenberg16]

I've found with my 300BLK load development that it can fall off a cliff pretty fast. Even .1 of a grain can sometimes make a load work great or not at all. Same with temperature. That is one of the parts of 300BLK that takes a little work. I usually run into the problem you had when I'm trying to load my lighter bullets subsonic. I think a similar issue is at play in that it could be affecting the pressure curve.


I will say this...110's run great out of my SBR with pistol length system, and I found that in general most of the "marginal" loads I came up with for my carbine length upper do tend to work better in the pistol length upper. You also mentioned an adjustable gas block. Not sure if you put it on yourself or if it is a factory set up, but any gas leakage could also affect cycling.


If you don't have any heavy bullets handy, you might play with seating depth a bit at your max loads so long as you aren't already seeing pressure signs. I found that on subs, seating depth also had a big impact on proper functioning and cycling.

 

[maverick18]

Most people on 300blktalk forums are NOT able to cycle 4227 with carbine gas non suppressed. Some barrels on carbine gas have too small of a gas port and a friend of mine had luck opening up his gas port to allow cycling with a larger variety of powders.

 

[devious169]

I've read about some people opening up the gas port,I'm trying to avoid reaming a new barrel. That's a last resort. I've checked the gas block no signs of gas leakage, the carbon ring is uniform with the gasblock. I'm going to try to work up to the max load starting @ 19 gr.- 19.5. And maybe a lighter buffer and spring. Thanks for all the input fellas.

 

[Broom_jm]

Even though IMR4227 is not too far away from H110/W296 or Li'l Gun, it's actually a slower burning powder. To put it in perspective, H110 is one of the slowest pistol powders, while 4227 is one of the fastest rifle powders. A faster powder will generate pressure sooner, which can be critical in a gas-operated semi-auto. With the powder market the way it is, I'd want a gas port opened up (or adjustable) to run with a wider variety of powders.

 

[Sniper 79]

I have a 16" Core heavy profile barrel, full auto bcg, carbine gas system. Shooting real nice with 125grn pills loaded with 17 gr of 4227. Never had any issues with locking back on last round. I use both P mags and GI mags. Having lots of fun with my rig.

If it was me I would try a different load or shoot a few factory rounds to see if it is the gun or the load combo. Shouldn't have to work on a new barrel. Make sure your mag is ok and manual lock it back.

Think it is a matter of finding the right load for the rifle. 300blktalk is a great resource.

Good luck and have fun with your adventure.