50 gr loads for a 1:7 AR

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steveNChunter

New member
I've recently been given 2,000 Speer TNT 50 gr bullets and I'd like to use them to make some general plinking/small critter loads for my AR's and my RAR ranch in 5.56.

My main question is can I push these bullets down a 1:7 barrel without them coming apart before they hit the target?

And would it be a load with enough pressure to reliably feed in a direct impingement gas system?

I've got some H335 and Varget on hand, but willing to buy whatever it takes to get the right load as long as I can find it.
 
I would keep them on the lighter side (that will still cycle the rifle of course), as long as you don't push them too fast you should be okay. Might not be the finest accuracy but if you have 2k you have nothing to lose by trying, they might surprise you. Both of those powders are worth trying, again I would start low and stay as low as you can that will shoot well and cycle.
 
Just to be on the safe side. Fire one on a cold bore, inspect barrel for an copper remnants. Fire a couple more and inspect again. Fire a few for a hot barrel and inspect. Listen to each shot and notice any out of place shots. A 16" barrel is probably ok but might be a whole different result in a 22" barrel. If they shoot poorly, buy a Thompson/Venture bolt rifle w/ a 1:12 twist. They are garunteed moa. It has a 20" barrel. Any excuse to get another rifle is a good excuse!
 
Check with Speer. They'll tell the maximum velocity they've tried at some given rifling twist. Use those two figures to get the RPM:

Where:

MV1 = Muzzle velocity of published fasted load in their test gun with this bullet in ft/s
T1 = Rifling twist of published test gun in inches per turn.
MV2 = Muzzle velocity of your gun to produce the same RPM as the test gun did.
T2 = Your gun's rifling twist in inches per turn.

MV2 = MV1 × T2 / T1


That will give you a safe number. You may be able to go faster, but you'll need to work up to see if the bullets start vanishing in a gray streak on the way to the target.
 
If I look at the Speer H335 reference for the 50tnt, I see max charge and a given fps they had from their Ruger (1:10???). If I plug the mv from Speer into the formula, it provides the max RPM for their rifle. Not sure I understand after that? I already know the MV to get their max rpm (assuming I haven't pressured out). I'm missing the point. Can you break that down a bit more? thank you.
 
In the first instance you use the rifling twist of the test gun; 10 inches in your example. In the second instance you plug in your rifling twist to get the MV that is known not to blow a bullet up.

I made it too complicated, so I rewrote it just to use the ratios of the twist to make the adjustment.
 
Thank you for the rewrite. I understand now (I think).
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Speer page 200 H335 tnt50 max fps=3262 from their test rifle using a 1:10
Lets say our rifle is 1:7
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MV2 = MV1 × T2 / T1
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Mv2 = 3262 x 7 /10 = 22834 / 10 = 2283.4
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Bullet RPM = MV X 720/Twist Rate
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Speers Rpm = 3262 x 720 / 10 = 234864 = Our Rpm = 2283.4 x 720 / 7 = 234864
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The muzzle velocity of ours to produce the same test gun rpm is 2283.4
Speer H335 start is 2975 (above our calculated margin of safety).
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If I did this right, then we need a different bullet or a reduced powder type like Speers AA-5744 shown below. While ok for bolt rifle, would this even cycle an ar bolt?
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I have not used a gas block before, but to stay w/ H335, would a gas block resolve this, or is this bad science?
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If this is going to far afield, you can skip the reply. This is fun stuff!
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Thanks again for the re-write.
 
You got my idea just fine. Keep in mind, though, that this is merely an RPM that is known to be safe with this bullet. It may not be maximum, though Speer says the TNT is not normally recommended for very high velocities, so apparently it does come apart easily. I think you are right to be concerned about cycling.
 
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