300 blackout short stroking help

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greyling22

greyling22

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My brother has a blackout that I'm loading for for him. I'm using 17 grns of h110 under a 125grn sierra bullet that (according to load data) should be generating 2100 fps. A nice supersonic load. It works great. He has a carbine gas system and carbine buffer/spring.

I liked it so much I bought a complete upper http://www.shop.masdefense.com/16-300-BLACKOUT-BARRELED-UPPER-GTHD-15-KEYMOD-SALE-163GHDERBU-LD.htm for myself. However, mine is short stroking. It ejects brass about 3:30 and some at the 5:30 positions, sometimes cycles and picks up the next round, sometimes it rides over the next round. It was short stroking every time with the H2 buffer in it, and maybe 30% of the time with a standard carbine buffer.

So probably I should have got a pistol gas system, but I didn't want to be overgassed with the supersonics I am shooting. But I can't fix that. Any other solutions? super light spring or buffer? drill out the gas port? sell it, possibly for a loss and start over?
 
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I drilled my barrel gas port.

Used a carbine gas system, recoil spring and buffer in a rifle length buffer tube. I cut a dowel and slid it down the tube to make it think it was a true carbine system (timing)

Shooting supers and subs have never had an issue.

My buddy and I built identical rifles (except furniture) and his has been flawless as well.

I went to the woods because I wished to live deliberately - Thoreau
 
My 300 BLK won't cycle well if I use an M16 BCG vs a lighter AR 15 BCG, and that is with a standard buffer. Not enough gas.
 
Your load according to the LYMAN 50th ed. is on the lower end for the Sierra 125gr bullet. (16.8-17.8 H110). You might want to try working up the load and see if your cycling issues improve. I would load 10 rounds in 2gr increments starting at 17.2 gr and see how it goes. You should not have any problems with a carbine length gas system shooting supersonic loads. If you approach maximum load and still have cycling issues then I would start looking for other problems with your setup.
 
-and of course I have the m16 bcg....:rolleyes:
-I always start light with my loads, but I'm going to try upping the load just a tad, maybe 17.4ish
-I also took apart my buffer (way easier said than done. my plastic insert was really tight in there) and took out 1 of the 3 weights in there.
-I have some GI surplus 150grn fmj's laying around that I use in the garand that I will try.

if that doesn't work I'll give clipping the buffer spring and enlarging the gas port a go.
 
I tried all kinds of recommended loads in a search for ammo that would cycle my first carbine length 300 blk upper. I ended up having to drill out the gas port a bit. Now, everything I put through it cycles properly.

For my 10.5" barrel with a pistol length gas system, I'll use an adjustable gas block. If necessary, I'll open that gas port, too. Life's too short to play mix-n-match with buffers, BCGs, springs, etc.
 
A 110 Gr V Max at 2200 FPS cycles my BLK.
A 125 Gr TNT cycles it at 2000 FPS.
A 147/150 gr FMJ cycles it at 1875 FPS.
A 155 Gr A Max cycles it at 1700 FPS.

Some of these were shot at a slower speeds working up and cycled the gun.

AR 15 carrier, standard buffer.
 
ok, got back from the range again. I took 1 of the 3 weights out of a standard carbine buffer and tried my new loads:
125grn sierra over 17.5grn of h110 = perfect function. brass ejected about 10' in the 4 o'clock position. great accuracy
150grn fmj over 15.4 of h110 = 2 strings of 3 shots. bolt locked back 1 out of 2 times. brass ejected about 7' in the 4 o'clock position. great accuracy.
160grn powder coat gas checked lead bullet over 15.4 of h110. = cycled fine, failed to lock back bolt. ejected 5' in the 5 o'clock position. great accuracy.
160grn powder coat gas checked lead bullet over 14.8grn of h110 = short stroke/fail to feed half the time, locked bolt back, ejected 5' in the 5:30 position. accuracy was not particularly good.

so, am I still under gassed? it seems like it. my gas port measured out .120, and I am as properly centered as I can get. do I need to be looking at a nemo or wolf reduced power spring? take another weight out of the buffer?

ultimately I'd like to be shooting that 160grn powder coat gas checked lead bullet. jacketed bullets are expensive.
 
I have a mid-length upper. It requires Acc#7 or Acc1680 to function. Max loads with a 125gr will work with max H110 loads, but lighter bullets, or heavier cast 155+, only work with medium to max loads of Acc#7 or #1680.
It's about gas volumn, and duration of pressure cycle.

But, I love the ergonomics of the mid length Magpul forend. It's worth it.
 
OK, I'm confused by that. #7 is a faster powder, and 1680 slower powder. I could see one or the other, but not both.
 
GooseGestapo said:
I have a mid-length upper. It requires Acc#7 or Acc1680 to function. Max loads with a 125gr will work with max H110 loads, but lighter bullets, or heavier cast 155+, only work with medium to max loads of Acc#7 or #1680.
It's about gas volumn, and duration of pressure cycle.

But, I love the ergonomics of the mid length Magpul forend. It's worth it.
Click to expand...
Mid length 300 BLK? are you sure? 16" barrel?

Greyling22, I saw you said you have H2 buffer, change back to standard buffer will help.
 
HankC said:
Mid length 300 BLK? are you sure? 16" barrel?

Greyling22, I saw you said you have H2 buffer, change back to standard buffer will help.
Click to expand...
I did change buffer. then I took a weight out of that buffer. life got better, but still a little problematic.

as far as opening up to .125, isn't the gas system only .120 in diameter? Does the larger diameter just help ensure you are lined up right?
 
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as far as opening up to .125, isn't the gas system only .120 in diameter? Does the larger diameter just help ensure you are lined up right?
Click to expand...

It helps to reduce "head loss". Pressure loss is significant when the gas makes a 90 degrees turn at the port. The gas block hole size should be larger than the port which helps alignment.
 
GooseGestapo said:
I have a mid-length upper. It requires Acc#7 or Acc1680 to function. Max loads with a 125gr will work with max H110 loads, but lighter bullets, or heavier cast 155+, only work with medium to max loads of Acc#7 or #1680.
It's about gas volumn, and duration of pressure cycle.

But, I love the ergonomics of the mid length Magpul forend. It's worth it.
Click to expand...
Interesting. I just used a midlength float tube over a low profile gas block. The 300 BLK barrels I have seen have a carbine gas set up. Do you know what diameter the gas port is?
 
Gas block alignment is trickier than first glance. The port is drilled with the idea that the gas block is spaced out from the barrel shoulder by a hand guard cap - which measures about .030". It can vary but the point is unless you check the holes may be misaligned if the block is just butted up against the shoulder and tightened down.

Drilling out the port helps because it removes the overhanging material blocking gas passage.

Different loads, different gas pressures. Varying the load might get the operating cycle into the pressure map but you are then limiting the combinations you could be using. And there is the very real eventual issue of gas port erosion, which will open it up anyway. In a couple of thousand rounds things may start changing because of it.

Further stuff: http://www.ar15.com/forums/t_3_12/585371_Gas_block_shim_.html
 
On mid-length barrel (it was given to me, found out why...!) I had it drilled out to .125" gas port.
RL7 is slower than 4198's or 1680. 4198's occupy too much volumn- too low weight, to generate sufficient gases volumn. 1680 works best, but RL7 is more accurate.

But, it shoots great with 155 gr Lee PTGC and 150gr FNGC.
 
Sorry about your problem, every BO i have built with a 16" barrel uses a carbine port, the 12.5" and 8.5" pistols use a pistol set up. I have never had a gas problem shooting bullet weights from 110 gr to 167gtr and with my standard 150 gr load i use 16.2 gr of W 296 or H 110 ( both powders same ballistics) i have foubd great accuracy. Still trying to find the magic combination for my Ruger American Ranchl. I have taken numerous Texas hogs with the Blackout round including one boat at 350 + lb boar (125 gr TNT 17 gr H110 Out of a Handi rifle which is now extinct).
Obviously all of my loads are supersonic.
 
every BO i have built with a 16" barrel uses a carbine port, the 12.5" and 8.5" pistols use a pistol set up. I have never had a gas problem
Click to expand...
+1. 16" carbine gas should do just fine especially supersonic, there should be no need to clip the buffer spring. It is likely there is gas blockage, leak or binding somewhere. But at least you get it run now!
 
I went through some similar problems with a carbine length .300. I pulled and verified alignment of gas block then drilled gas port next size drill up in my index of letter bits. Still short stroked. This was with 150gr Fiocchi that runs fine in a Midway carbine .300 I have.

I didn't mess with the buffer or spring but I did swap a semi auto BCG from another rifle I had for the M16 BCG and that proved to be the trick on mine.
 
Same for me, it needs the lighter semi auto (AR 15) vs the heavier (M 16) carrier.
 
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