Short Stroking in Subsonic AR15

[RobRat]

I had a local gunsmith build an AR15 upper for me in 300/221, and a matching barrel on a Savage bolt action. Specs on the AR barrel - 16.5", 1:8 twist McGowan barrel, TP555 3 position gas block in the pistol position. The lower is equipped with a commercial 6 position stock, and DPMS GI trigger. I want to change both of them eventually and get a better trigger and a stock thats better for shooting off the bench, possibly just an A1 stock.

I had it threaded for 5/8 threads, and supplied a YHM suppressor mount to install on it. My intension is to buy a YHM 7.62 suppressor and share it between the 2 rifles- -the mount that comes with the YHM Suppressor will go on the Savage.

As it happens friend of mine has a YHM 7.62 suppressor, and came down to shoot it with the 300/221. I'm still smiling.....suppressers and subsonics are awesome! A suppressor is now on my VERY SHORT LIST of things to buy! (...and a 300/221 is on my friends short list now)

My issue - It cycles fine on supersonic stuff in the middle gas position, but when shooting subsonic loads in the full open position, it was short stroking. Very close to functioning properly though, as it ejects with athority, and resets the trigger, but the bolt is just not coming back enough to strip the next round off the magazine. The bolt fully closes, and you just get a click when you pull the trigger on the second shot.

I'm using VN110, under a 240gr SMK, Fed 205 primer. With 9.0 and 9.3 grains of powder they were coming out subsonic, at 9.7 grains they were supersonic. According to the Sierra loading data, 9.7 should be 1050FPS, 9.3 should be 1000fps, 9.0 around 960fps.

Would a different powder help? If so, which should I try next? I'm assuming I would want a faster powder? Would a lighter buffer help? Which one? Can you cut the buffer spring down?

[SgtCottle]

I wouldn't do any heavy modifications to the lower at all ... it isn't needed if the barrel is set up correctly. First ... do you know what your gas port was drill out to be? Most AR type rifles have them drilled to .078" and opened up as needed. With the 300 Whisper line ... you should increase the gas port to .120" to allow the full amount of gas through and just use your adjustable gas port to regulate how much is actually used. This would be the first thing that I looked at because it affects all your other decisions, and it is the easiest to fix.

With my 300 Whisper I have found that N110 is a little too fast for some of my loads, and does not produce enough gas to cycle the action. You will actually want to go with a slower burning powder to increase the amount of gas produced. Try Hodgdon H110, Lil' Gun, Accurate 1680, or even Reloader 7 ... all these are a little slower than N110 and produce a little more gas. This should help cycle the action with no issues.

Since you were right on the edge of having a complete cycle I bet your gas port is opened up, but I would go ahead and visually confirm it anyway. It may resolve problems down the line. With it being so close already I bet any of the first three would cycle the action with Reloader 7 allowing you to use even lighter bullets for subsonic practice (I use a 200g SMK with Reloader 7).

Hope this helps.

Sarg

[TCCrewchief76]

I've got a gas port in the pistol position of .118" diameter. A buddy and I just shot it this weekend with some 220 SMK's and 8.4 grains of Ramshot Enforcer (suitable name for the powder given the intended purpose of the cartridge ), and velocity out of the 10.5" barrel was right at 1000 FPS. Function was just fine, with the empties falling about 6 feet to our right. The weapon was wearing a GemTech HVT, for reference.

Happy To Help,

Kevin

[mak91]

I have a 10 inch barrel with the same TP555 adjustable gas block as you in pistol position. With it opened in the wide open position it will cycle 220/240 smk under 8.2 grains of LiLGun.

[i8asquirrel]

Sarge Is giving good info, I was having same sort of issue with 16" barrel and pistol length tube . Opened it up to .116" and lightened buffer. it works like a champ now....switching to A1680 and IMR 4227 powder was the ticket.

[RobRat]

Update - its working better....

I switched the lower to a A2 stocked lower w/rifle buffer, and got short stroking about every 3 rounds. I'm going to try to load up some rounds with AA1680 (if I can ever find any- -none of my local shops have it, so I'm waiting on an order to come in). Seems to work better the more I shoot it.

I ran out of my subs, and then switched over to some 125gr and 110gr supersonics, and switched the gas block to the restricted setting, and it ran 100% there. Had some verticle stringing with the 110's (a 2-1/2" long verticle line at 100), while the 125's produced 1-1/2" round groups. Back to the reloading bench....

Until my suppressor shows up (hopefully it will get processed and in my hands by Christmas) a question...Does a suppressor create more back pressure, and should make the rifle cycle better with subs?

I'm going to try my RRA 2 stage A2 lower, and see if a better trigger makes for better groups- -I swear the GI trigger I shot with tonight creeped an inch before it broke.

[Titleiiredneck]

a question...Does a suppressor create more back pressure, and should make the rifle cycle better with subs?


yep, i suggest a pri gas buster latch also.

[buffetdestroyer]

A suppressor should create an increase in back pressure.

I would also get a chronograph to test the speed of your N110 loads as you may be slower than you think with your current load.. I estimated mine and was about 100 fps faster and my load definitely doesn't correspond to the 14" Contender barrel loads from a velocity standpoint.

Each barrel is a law unto itself with how it was cut, total number of grooves, chamber dimensions etc. so the manuals are a great starting point, but you can save a lot of load development and trying new components by knowing your velocity for certain. Additionally if you are too slow, you may run into issues with squib loads and stuck bullets which ruins barrels and body parts if not caught!
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BIRTH CONTROL ADVICE

[love2shoot]

Originally Posted by Titleiiredneck

a question...Does a suppressor create more back pressure, and should make the rifle cycle better with subs?


yep, i suggest a pri gas buster latch also.

Yes, it would make a difference. couple weeks ago I was doing some initial sub-sonic load testing. had some loads that wouldn't quite cycle it. the carrier would catch on the BHO, but not the bolt......added the suppressor and it ran fine.

I used that info from those loads to find different powders to give me the port pressures I need.

[Fritzcat]

RobRat, the middle position on the gas block should be for sub-sonic, the rear position is for super-sonic, and forward for single shot. The gas block should have a C closed, M moderate, and H high. You can also verify by using some small drill bits and insert them in the gas tube hole. My middle position is .125. I am able to shoot 150 FMJ using Unique or Blue Dot. The 240gr and powder you have should shoot just fine.