03 July 2017

Empirical

A long time ago...

I took Sabrina out shooting.

Because she's a 1:9 twist and an 11.5" barreled AR, I'd decided that conventional 55gr FMJ was insufficient and had been using 69gr Sierra Matchking (SMK) ammo from Black Hills.  The SMK outperforms milspec ammo at most things from what I've read.

The down side is that it's expensive and not readily available on gun shop shelves.

Luckily I discovered that it shoots to the same point of aim as M855 and XM855 ammunition which is cheap and readily available.

Unhappily, most of the indoor ranges ban M855 because of the steel core.

So I was shooting my "war-stock" SMK at the range and having fun until I'd run out of ammo.

Because I wanted to keep shooting, I bought some PMC 55gr FMJ from the range.

This did NOT fire to the same point of aim as the SMK at all!  Like 9" different low and to the right.


Because of this graphic, I just assumed it was normal and vowed to not shoot 55gr in guns that were zeroed for SMK.  (but notice that low and LEFT is what you should get with a rifle zeroed for M855 shooting M193)

Erin, on Facebook, asked her more experienced friends for their experiences with different twists and different bullet weights and I related my range trip.  I was told my assumptions were wrong and something else must be to blame.  I promised to get back to the range and see if I could duplicate the experiment.

I am here to say that could not.


That's Federal XM193 55gr FMJ at the same 25 yards which gave a 9" difference before.  This is firing slow, off hand, supported and fast...  My eyes are making more difference than the bullet weight and twist.

So, what's different?

She's sporting a different barrel nut and flash-hider than before.  This is because she changed from being a GAU-5A to an XM177E2 and that meant changing uppers.  I ruined the barrel nut when I slipped while torquing it down.

I think that the old nut was out of spec because it'd always seemed to take too much torque to line up the gas tube.  The new nut lined up at about 50 ft-lb.

The old muzzle device was very open inside:


The new, and current one, is more closed:


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