Feed issue with a wrinkle

[wrbabb]

I have 2 AR's - one A2 .223 and 6.5 Grendel.

A2 lower is a Rock River. With the 6.5 Grendel upper it functions virtually flawlessly. Feeds and Magazine lock back.

Grendel lower is a Surplus Arms lower - admittedly went for a low bid. Anyway I am experiencing feed failure and intermittent lock back.

What on the Surplus Arms lower could need adjusting? I have not put the .223 upper on this lower to see if it has the same problems.

Thanks,

Walt

[LRRPF52]

Buffer weight and recoil spring is where I would look first.

Next problem could be something else, like mag catch indexing too high or too low.

Pull your buffers and keep them apart. Weigh them and report back.

Also, keep the recoil springs separate. What's the length of the recoil springs?

[terrywick4]

I would swap buffer and spring between the two and see if it fixes the problem.

[wrbabb]

Great I am on it. Thanks.

[wrbabb]

Results of Buffer weighing and buffer spring length

Both buffers weighted 5.125 oz - same.

Both buffer spring lengths 12.625" same.

I suspect it is the lower or working components in the lower - mag release bolt catch or such.

Walt

[Keep The Change]

Try putting the 223 upper on the "problem lower" and see if it works.

This might confirm a mag catch issue in the lower.
What mag are you using for the 6.5G?

[wrbabb]

As suggested I put the 223 upper on the SAA lower - same problem except I was using Magpul magazines. When I switch to a Brownell's and a Colt steel Magzine lock back and feeding was was resolved.

On the A2 lock back worked on all magazines. Using a dummy round(s) I was able to get feeding on the metal magazines in the 6.5.

Adjusted the tension on the magazine release no change - Did not expect it.

So If I zero in the lock back as an issue how can I adjust it or should I replace it?

Walt

[LRRPF52]

Measure the distance of the magazine catch slot on the lower to the top edge of the upper with calipers.

Compare with the lower that works.

[wrbabb]

Both measure .1760 from center of pivot pin to top of the lower.

The next step would be to swap the latches between the two. For further inspection they both will have to come out anyway.

That said, the steel mags work in both. The magpuls only work in the A2 (Rock River lower). I may work on the rear area of one of the magpuls to see if that will resolve it. There is quite a difference there between the steel and magpuls.

[VASCAR2]

What buffer tube are you using with Surplus Arms lower. You mentioned you had an A2 stock on your RRA lower, I assume you mean an A2 fixed stock. What type of stock buffer tube are you using on the Surplus ammo lower?

[LRRPF52]

Do you have 6.5 Grendel magazines? Magpul should not be part of this equation, unless you are just testing functionality with the 5.56 upper.

[wrbabb]

Unbranded but it is a standard A2 buffer - fixed stock. The stock is a Magpul with adjustable length-of-pull and cheek rest.

Right now I have compared the mag latch between the two and the one on the Surplus Arms is different in that the thickness on the top to where it rests on the lower is less. Hard to describe. I am off tomorrow to find another - hopefully a RRA latch or least a thicker one.

[VASCAR2]

Unbranded but it is a standard A2 buffer - fixed stock. The stock is a Magpul with adjustable length-of-pull and cheek rest.

Right now I have compared the mag latch between the two and the one on the Surplus Arms is different in that the thickness on the top to where it rests on the lower is less. Hard to describe. I am off tomorrow to find another - hopefully a RRA latch or least a thicker one.

Looks like your running a carbine buffer tube with collapsable stock. If you have a carbine stock/buffer tube you need to use a carbine buffer and spring. The standard carbine buffer is 3 ounces and the standard carbine spring is 11.250" long. They make heavier buffers for use for over gassed uppers but if your running a 20" or longer barrel they are rarely over gassed.

If your running a rifle length spring and buffer in a carbine buffer tube your gun would act like it was under gassed. The AR-15 could be short stroking, bolt not going far enough back to feed the top round in the mag.

What length barrel do you have in your 6.5 Grendel upper? If your using a collapsable carbine stock you need to use a carbine buffer and spring even if your barrel is 20"-24".

[wrbabb]

Same buffer tubes/springs on both. A2 fixed stock type.

[lawndartmike]

One thing that had me going in circles was triggers. The amount of drag it puts on the BC will affect cycling. I have a 7.62x39 upper that functioned flawlessly on it's lower with the Ruger trigger. I moved the trigger to my Grendel's lower and put a mil spec in the 7.62 lower. All of a sudden no lock back on empty mag and a few failure to feeds. Bought another Ruger trigger and it is back to functional again. Looks like the lighter spring on the Ruger trigger put less drag on the BC.