7.62x39mm AR ballistics surprise


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MMcfpd
September 6, 2008, 10:06 PM
Both of my 7.62x39mm ARs went to the range this morning, one a 16" Model 1 kit gun with an Aimpoint M4 and the other built with a Model 1 barrel that's been cut down to 14.5", with an Aimpoint T-1. Their barrels are both chrome moly with 1:10 twists.

Both were zeroed at 50 yards, and I decided to try 'em on the 100 yard. The 14.5" is new and I hadn't done that with it, and I don't really remember the last time I tried the 16" on the 100.

This may well be old hat to the more ballistics savvy amongst us, but it was a discovery for me. Firing Sellier & Bellot 123 gr. FMJ out of both guns, the 16" was hitting pretty consistently about 2" high while the 14.5" was pretty much on the bullseye (as much as I am on the 100 - the altitude was right, but I still find accuracy with a 4 MOA red dot at 100 yards to be a bit challenging).

A better explanation from those who know is welcome, but my best guess is that the slightly lower velocity that round gets from the 14.5" barrel allows that 123 grain bullet to drop, compared to one flying out of a 16" barrel, enough to be pretty much flat at 100 yards with its 50 yard ballistic profile.

Anyway, the outcome surprised me a bit. I shot a couple of mags out of each and results were unchanged.

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wideym
September 6, 2008, 10:12 PM
I've been looking at buying a Colt 7.62x39 AR, but it comes with a horrible looking frankenstien mag. The mag is a modified AK mag made by Promag just before the AWB. A friend had friend had a couple of those Promags for his 7.62 AR, but all the mags failed horribly.

My question is what type of mags do you run in your AR? Are they reliable, how many rounds do they hold, are they available, and at what cost? Any help would be appreciatted.

MMcfpd
September 6, 2008, 10:26 PM
In one 7.62x39 AR I use the 20 round USA frankenmags with fair success - they often don't feed the first round completely and a second pull and release of the charging handle is required. My other 7.62x39 AR's magwell doesn't accept those mags as readily (mag catch doesn't reliably catch).

In both I use the 10 and 30 round C Products (http://www.cproductsllc.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=cp&Category_Code=7_62) magazines, and they work just fine. The 10 rounders have been out for a while now, and I tend to use them more from the bench, because the 30s are long.

C Products struggled for quite some time with the 30 rounder, but it appears the new ones work fine. They're still fairly new, so I guess they're still a bit unproven.

Also, a standard 5.56 mag will work with 7 rounds.

macadore
September 7, 2008, 12:52 AM
How does the accuracy from the AR compare to the 5.56 in the same platform?

HB
September 7, 2008, 12:55 AM
How does the accuracy from the AR compare to the 5.56 in the same platform?

It should be the same if ammo quality is the same. Talk to Krochus about his 7.62x39 benchgun

HB

R.W.Dale
September 7, 2008, 01:01 AM
I'm flatteredhttp://www.100megsfree3.com/fwc493/SmileyDirectory5/B/C_EmbarrassedR.gif

oddly enough I'm about to build myself a 30 cal AR quite possibly 7.62x39mm

macadore
September 7, 2008, 01:14 AM
Thanks HB. I often see the accuracy of the 5.56 AR compared to the 7.63x39 AK. That hardly seems like a realistic comparison of the two cartridges.

Z-Michigan
September 7, 2008, 02:52 PM
I would not expect enough difference in velocity out of 1.5" of barrel length for that 7.62x39mm cartridge to have a really detectable effect on the 100yd impact vs. 50 yd zero. I would be curious to see chrono data on both rifles but I'm going to guess the 14.5" is shooting only about 20fps slower.

My first question - you're using two different aimpoints, what is the sight height above bore axis on each one? It's possible a significant difference in sight height (vs. bore axis) could cause this.

Second idea - depending how large or small your groups are, it could be just random chance.

Try this ballistics calculator:
http://www.handloads.com/calc/index.html

Use 123gr, I would guess 2300fps, a ballistic coefficient of 0.28 is about right for FMJ ammo of that weight and caliber. I varied the sight height from 2" to 4" using a 50yd zero for both, and the impact difference at 100 yds was exactly 2". Maybe coincidence, but I would check the sight height over barrel on each rifle.

In comparison, if you leave the sight height at 3" (about right for typical optics on a flattop AR) and go from 2300fps to 2200fps, a much bigger reduction than I would expect, the additional drop at 100yds (vs 50yd zero) is only 1/8".

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