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. :smile: 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 :smile:), 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.:smile:

[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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[love2shoot]

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.

[RobRat]

Checked my TP555 gas block, mine is (going clockwise) closed, then partially open, then fully open. Talked to the 'smith that installed it, he didn't remember the size of the port, but he says he drilled it to the size that the gas block came with. He's a real particular machinst, I doubt that he has it misaligned, but I may pull it and check if some further attempts don't pan out.

I tried lightening the rifle buffer - started at a stock weight of 5.2 oz, and dropped it to 4.5 oz by removing 2 of the weights and replacing them with a bronze bushing. Going to try taking more out.

I ran out of 240's and could only find 200's locally. :mad:

Tried 3 local gunshops, no one has AA1680, and the shop I ordered a pound from hasn't been to the distributor to pick it up yet, but should happen soon. :mad:

In the meantime, I'm trying some H4198 with the 200 gr smk's. Anyone tried H4198 in their Whisper? I've loaded 10 test loads (3 each) from 11.0 to 14.6 gr, stepping by 0.4gr. H4198 is #58 on the burn chart, AA1680 is #54, and VN120 is #59, so it should be in the ball park. I heard powder crunching when seating with the 14.2 and 14.6 loads.

[RobRat]

Tried the H4198 under the 200SMK's - and it cycled wonderfully! The slower powder is doing the trick, If I can only get some AA1680!!!

The range was crowded, and I was only able to set the chrono up for 4 of the loads, and shoot 3 of each.
11.0 gr averaged 857 fps
11.4 gr averaged 925 fps
11.8 gr averaged 1017 fps

I was able to chrono one other load - 13.0 gr, and it was supersonic with an average of 1183 fps

Because of the range situation I was not able to both test accuracy, and velocity at the same time, but I wasn not impressed with the accuracy of the loads I did shoot at targets - they were good horizontally, but at 50 yards had an inch and a half of horizontal.

[ezed]

i read your initial post and had exactly the same problem that you did with several loads last weekend. My whisper is set up pretty much like yours. The load that funtioned perfectly in mine, was 11.7 gr of 1680. average velocity for 17 shots was 1018 ft/s. outdoor temp 32 degrees
aa9 8.7 gr same as you described
aa9 9.1 gr same as you described
h110 9.6 gr same as you described
i will try more loads but this worked for me. email me if you want more complete data.
have fun!!

[ezed]

I just tested 4 different loads last weekend thru my 10.5 whisper.
220 gr smk
tp555 gas block subsonic position pistol position
gas port .115
Gemtech Sandstorm suppressor
ambient temp 32 deg
I had the same problem as you did with three different loads: extraction great, ejection great, reset the hammer but would not pick up and put a round back in the chamber. Pulled the trigger and the hammer would fall, but....
Loads that did not work:
AA 9 9.0 gr average velocity 1101
AA 9 8.7 gr average velocity 1022
H110 9.6 gr average velocity 1044
The load that worked and cycled perfectly:
AA 1680 11.7 gr average velocity 1018
email me if you would like complete data and information. I have each load on an excel spreadsheet.
I saw that someone posted a load for H322 somewhere, but cannot find that information. I figured that could be a good load cuz it makes a carbine work. Ok, strange, but, what the heck. Why not try it? Anybody have a load for 322?