[tpe187]

Hello All,

Well, I was trying to conduct some research into the 6.8 and compare it to 5.56 for speed and shootability in standard drills for possible military application. So, I ordered one of Noveske's new barrels in 6.8. It has the Mod 1 chamber, which is their version of the SPC II and is 14.5" Originaly I built it up on an LMT upper reciever with LMT carrier and an older gas tube. Bolt is a CMT/STAG 6.8 bolt since Noveske accidently sent me a 5.56 bolt with the barrel.

The problem: I get intermittent short-stroking. Here is what I have done.

Magazines: 2 PRI, 2 Barret, 3 Cproducts. Load one and shoot to check for bolt lock. I might fire 5-10 before it will do it, but it does it in all magazines.

I have used two differnt bolts, both CMT/STAG, two upper recievers, BCM and LMT, 3 gas tubes, two Colt cam pins, lots of lube -SLIP2000 EWL. I have pulled the barrel off and checked the gas block and gas port for obstuctions. Nothing and no sign of excessive gas leaking out. Gas port measures .070 with a caliper. I've tried two different lowers and three weights of buffers, H, H2 and H3. Action spring is new colt in one, LMT in the other lower. Still does it.

Ammuntion: Reloads - SSA small primer brass, new and once fired. 110gr Hornady OTM, Light crimp with LFCD, 30.1gr H322 over a Rem 7.5 primer. Avg Velocity is 2638 for 15 shots on an 80 degree day. SD12.4, ES 47

I've done the standard checks. No unussual wear marks in either reciver where the cam pin might be rubbing. Gas keys are tight and properly staked. Lube is generous. Chamber is clean and dry. Normally I would think this was an ammo issue, but that velocity should be more than good enough to cycle the action reliably. So, hopefully I didn't miss anything.

Anyone care to comment on what might be the problem? I'm really at a loss on this one and I'm pretty familiar with AR's.

My next step is to run all shots over the chrony to see what the veloicty was when it short strokes.. Round count right now is about 300 down the bore. After that I'll have to suck it up and try some factory ammo. I was hoping to run this for one day in an upcoming Pat Rogers carbine course, but it looks like it won't be ready. Any thoughts are appreciated. Thanks

Tom

 

[paulosantos]

Welcome aboard. I would use the regular carbine buffer with the regular carbine springs. If that still doesn't work, I would open up the gas port.

 

[HTR]

-I wholeheartedly agree that anything other than the H buffer and standard spring will only make the SS more likely.

-Are there any marks on the brass? Any linear striations on the brass? Did you measure the shoulder and case neck dimension? Is the top of the case shoulder 1.414" from the rim? SSA brass has recently been reported to have neck diameter beyond the nominal .304". Some cases have measured as big as .307" at the junction of the neck and shoulder. If so, these woulld be "sticky" when attempting to remove them from the chamber by manually ejecting, or they will lock the bolt altogether.

-Have you tried hand-cycling your loads to see if they ejected very easily from the loaded/chambered rifle?

-30.1 grains of H322 under a 110 grain bullet should definitely cycle the action. If you do buy factory ammo, I would try the SSA Tactical loads. They are a little higher pressure. I would try the SSA Tactical 110 grain TSX, the 110 grain Accubond, and the 115 grain SMK.

-Be aware that Remington and Hornady factory ammo are among the lower pressure and velocity loadings and will not give you a good representation of the potential to cycle in such a new rifle.

-The above mentioned SSA tactical loads should cycle just about any AR action.

 

[tpe187]

Thanks, I'll check out the standard carbine buffer. I did size my brass before loading the first time, but I'll check out those dimensions you listed. Most of the rounds fired were from the initial batch of 300 new cases so that is one possible cause. I'll get back to you guys on the results.

 

[Lastrites]

When in doubt take a run with factory ammo, just in case your missing something.

 

[rushca01]

Thanks, I'll check out the standard carbine buffer. I did size my brass before loading the first time, but I'll check out those dimensions you listed. Most of the rounds fired were from the initial batch of 300 new cases so that is one possible cause. I'll get back to you guys on the results.

What kind of die set did you use?

Reason I ask is b/c Noveske uses their propritory MOD 0 chamber.

Continuing on what HTR said about the neck size you may also want to check what die set you are using. If you are using a full length die set that may be causing some problems with rounds "sticking", the small base sets seem to work best with the auto loaders.

 

[-k-]

You did not mention replacing the most common cause of short stroking, the bolt carrier. Try a known good carrier if you have one.

What trigger are you running?

 

[HTR]

"K" makes a good point about the carrier, but I presumed that you had changed to a different one when you said this (yes, I did the unthinkable and assumed that by "bolt" you really meant BCG ):

"...I have used two different bolts, both CMT/STAG, two upper receivers, BCM and LMT, 3 gas tubes, two Colt cam pins, lots of lube -SLIP2000 EWL. I have pulled the barrel off and checked the gas block and gas port for obstructions. Nothing and no sign of excessive gas leaking out. Gas port measures .070 with a caliper. I've tried two different lowers and three weights of buffers, H, H2 and H3. Action spring is new colt in one, LMT in the other lower. Still does it...."

The only time I see problems with the full length dies is when the brass is not being sized to exactly .420" or smaller. The small base dies will size to .418". I am using RCBS full size dies and my brass is perfect in three rifles. It is always sized to exactly .420". If the Noveske chamber is .422", which I expect it is, you should not have brass sticking secondary to the sizing of the case body and base.

 

[tpe187]

Sorry, I knew I forgot to add some things.

Yes, two bolts - CMT/STAG and two different carriers - LMT and BCM. Both are properly staked and known to work in my other AR's

I'm using Forester dies for the reloads. I have been thinking about the small base dies as recommended here and I'll order a set, just to be sure.

I'll check the trigger, but I think its a standard LMT in the one rifle and a standard Colt in the other.

 

[Lastrites]

Checking the trigger for drag marks on the carrier is a possible binding area. You got an extra lower to try out? I'd still try some SSA factory ammo.

 

[Clint]

Welcome aboard TPE.

Did you check the angle of the ejected cases?

This will tell you about your bolt carrier speed.

It can be too fast or too slow or inconsistent.

And just to be clear,what exact problem is it having?

No lock back on single rounds?
Bolt over base jams?
Fail to eject?
Fail to feed/ bolt closes on empty chamber?

Also, I wouldn't mind hearing a little about your research if you can share...

 

[tpe187]

The ejection pattern is good. Cases eject at between 3 and 4 o'clock about four feet away. Cases aren't dinged either.

The malfunction is an empty chamber when firing a string of 5 to 10 rounds. This occurs off the bench when using the mag as a monpod as well as when running drills with nothing touching the magazine.

The carrier also fails to lock back periodically when testing mags with one round for lock back.

I checked my brass and that seems to be ok. Neck od is within spec. Cases manually extract without effort.
I cleaned the chamber and could not see any imperfections.
I blackened a cartridge and hand chambered it. Upon extraction there were no marks on the body, neck or bullet.
When chambering from a magazine I do get one long striation from the base to the shoulder. I figure thats from the feed ramp.

I will try the standard carbine buffer and one other lower before I go to differnt ammo. Kind of a bummer right now.

As to my research, I'm working on a masters degree and I could pick a decent topic. Basically, my thesis is that our small arms equpment, doctrine and training are not adequate for operations in Afghanistan where engagements occur beyond the 300 meters most units train to. 5.56 fragmentation is limited to under 200 meters from an M4 and M249 which further compounds the problem. Our SDM doctrine sucks in that it specifies some fairly basic instruction but does not standardize a rifle for his use. I've seen M4's with ACOG's up to M14's with 3.5x10 power Leupolds.

As part of my discussion I talk the merits of the 6.8 vs others like 6.5 Grendel, heavy 5.56 like MK262, or 7x46mm ARC. To compare the effect of higher recoil from 6.8 I've run some standard drills to get a baseline time for 5.56 and compare. I would hardly call the results conclusive, but I consider the 6.8 extremely managable and worth the reduced capacity and extra weight.

 

[rcrandall]

The 5.56 M855 62 grain FMJBT is the current issue round for the U.S. Military. The fragmentation threshold for that bullet is under 2700 fps. The muzzle velocity of the 14.5" M4 with this ammunition is 2900 fps. Running the numbers on JBM trajectories puts the velocity under 2700 fps somewhere around 80 to 85 yards (using 4000 ft elevation).

The SSA 115 grain OTM has been deemed legal to use by the JAG because it performs like a FMJBT when it hits ballistics gel, it yaws between one and two inches after penetration and breaks (fragements). This round chronographs at 2500 fps muzzle velocity from my 14.7" barrel and has a fragementation threshold of 1900 fps. Running the numbers on JBM the velocity falls below 1900 fps around 270 - 280 yards.

 

[Test_User]

A couple of things to try that I did not see mentions so if it was done or already mentioned please ignore:

You said you checked the gas port and GB. DId you check top make sure that the distance from the boss to the gas port on the barrel matches up with the distance from the edge of the gas block and the its hole. Sometimes gas blocks have the gas port drilled in them at a length from the edge that is in the same as the distance from the barrels boss ( the ledge the GB seats against) Sometimes this covers the gas port slightly.
I did not see where you mentioned the gas block type but if its a set screw: That the one out that is inline opposite the gas port in the GB. SPin the gas block around on the barrel and see how the barels port lines up in the set screw hole when its fulley seated against the boss ledge. If its clsoe to the edge given the threading of the set screw hole makes it larger its likely the gas port holel is partially covered. If its a dif type of gb you wil have to measure to check another way. The other possibly and one that was told to me by Constructor is to check the hole in the gas tube as sometimes they drill them in the wrong spot and they do not line up correctly with the hole in the GB.

Swap out the carrier or if you have another upper try it in that. Maybe a friend local has a carreir he can let you try for function. As that is the one thing you have not swapped out its worth trying.

Do the bolts move easily inside the carrier. (You can move it in and out with two fingers with fair ease.)

The gas port sounds to be the correct size for a carbine location.

I have found that the most common reason for SS in self built or other than manf built uppers is the gas block to barrel gas port holes are not perfectly inline. Many may look correct by eye ball but it the drilled port on the barrel is a hair off one way and the upper pin slot has it slightly off the center a bit then what looks to be dead center for the gas block in relation to the receiver and HG is not quite right with the gas port itself.

The other thing is if there are drilled dimples for the set screws. if they are slightly off in relation to the gas block and the port it could be pulling it off as you tighten them up even if you have the GB correct before you start to tighten them up good.

The good thing about SS is there is only some many things in the line.

Am I correct that it has SS with all the various mags so we know they are not the issue.

As this is interment and a new upper I suggest you try hand cycling the upper for 250-500 cycles as well.

Honestly from the sound of this it really sounds as if you have a slight mis-alignment of the gas port and GB. It only has to be ever so slight to cause this effect. You need to make sure side to side and front to back that those holes line up. If for some reason it looks close take a dremel and the cutting end ball attachment and open the face of the hole in the GB slightly all the way around as if its a small miss that will free it.

Report back on your progress.

 

[Bushman_269]

This is a totally serious question and not intended in any way to insult your shooting abilities. Have you been consistent in your shouldering and holding of the rifle? Has anyone else tried shooting the rifle and experienced the same stoppage? With a newly built tight rifle you can sometimes induce short stroking by not holding the rifle tight enough to your shoulder. Basically the long arm version of limp wristing a pistol.

 

[tpe187]

Bushmand - don't worry about it, after all I've covered I have almost run out of options. Yes, I had another shooter, An NRA Highmaster Service Rifle and long range shooter. He had the same problems, and I didn't say anything to him until he mentioned it closed on an empty chamber.

I think I might have it figured out. As per the recommendation to check the gas tube pin location, I checked it with other tubes and it was the same. The gas block lines up directly over the gas port and is pinned to the barrel. What I did find is that the pin location in the gas block is slightly off. Not anyting like a total obstruction but maybe 1/6 the lenth of the tube opening past perfect alignment. I'll see if I can snap a picture for you. I have a Larue gas block that I'll try out and see if it works. If it does, I'll either leave it alone or sent the Noveske barrel and block back to have a new block with the correct pin location installed. Thanks for all the help. I might have an answer tomorrow. Fingers crossed.

Tom

 

[OC455]

I had a similar problem with my 5.56, the gas block was off slightly and I was using a H2 buffer that I had. Changed it out to a standard carbine buffer & had the GB aligned and it ran perfect. That and the ammo I had, I changed out because of several times the primers would blow out and cause problems with functioning in the lower. Good luck!:a35:

 

[constructor]

excuse me if I blew past it but what length gas system? with a .070 port I am guessing a carbine?
if all else fails and it only misses the lock back every now and then upping the gas port by .005 should do the trick without over gassing the system.

 

[tpe187]

Its a carbine length gas system. I hope to get to the range tomorrow or sunday and test the different gas block with the standard carbine buffer. I'll let you guys know.

 

[paulosantos]

Since your gas port is .070", you really shouldn't be having any short stroking issues with it.