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Old November 12, 2019, 05:33 PM   #1
FizzyP
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Build a versatile .308: gas systems & hand loads

(Apologies: I'm asking this question here because it straddles a lot of categories and I expect "The Smithy" to have the most educated people who might have enough experience to provide a good answer)

Let's say I build a .308 AR in such a way that it would be heavily over-gassed (suppressor, light bolt carrier group, maybe a short gas system) except for the fact that I put an adjustable gas block on it so I can dial back the gas and make it operate properly. Suppose it's one of the click-adjustable varieties like Odinworks or SLR Rifleworks makes that have ~20 positions.

Assume that on click ~3 (out of 20) the action runs great with standard power .308 ammo. Now, if I reload weaker ammunition I expect I will have to open up the gas to keep it cycling reliably.

Let's say I keep reloading weaker and weaker ammunition until I get to the point where I have to adjust the gas block nearly all the way open for reliable function.

How weak are my loads at that point? Say I'm shooting 125 gr bullets out of an 18" barrel? How low do you think I can get the velocity? Can I get all the way to 7.62x39 performance?

To push this idea even farther. What if I built a rifle in such a way that it was over-gassed without a suppressor but it ran well when I turned the gas way down. From the previous thought experiment I have loaded weak ammo that runs reliably with full gas. Now if I put a suppressor on it pressure will be increased and I should be able to load even weaker ammo and get it to cycle reliably. Again, how weak are my loads at that point? How low can I get the velocity? They'll certainly be less powerful. It's only a question of how much.

Maybe this idea is far-fetched and won't work (which is why I'm asking) but surely you can see the appeal: 1) build an over-gassed, super-light .308 AR, with a suppressor and a click-adjustable gas block. 2) End up with a rifle that behaves like both a thumper .308 battle rifle and also a soft shooting suppressed modern sporting rifle in an intermediate cartridge with nothing but a few clicks on the gas block and an ammo swap.

Other important questions: can you foresee any other problems with this idea, and, if so, can you think of different way to accomplish something like I'm describing?

Thanks for any advice.
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Old November 13, 2019, 11:53 AM   #2
FizzyP
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Maybe a better question is, where is the right place to ask this question?
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Old November 13, 2019, 12:54 PM   #3
T. O'Heir
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"...How weak are my loads..." They're not. They're just slower due to the lack of barrel length.
"...all the way to 7.62x39 performance..." The lack of 7.62x39 performance you mean. A Max load for a 125 out of a 24" .308 Win runs around 3000 or 3100 FPS. A 7.62 x 39 Max load runs 2200ish to 2,554 FPS, also out of a 24" barrel. The whole thing is about case capacity.
Anyway, if you go too low on velocity you will have unreliable functioning due to the load being too light. And don't even think about loading below minimum loads. That can be dangerous.
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Old November 13, 2019, 02:56 PM   #4
FizzyP
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It seems like you're responding to the sentences in bold without having read the rest of the post.

Quote:
"...How weak are my loads..." They're not. They're just slower due to the lack of barrel length.
There are no loads. This is all a hypothetical question about how to build a versatile rifles. Being able to reliably shoot both standard .308 and weak loads with light bullets is the whole goal. I'm not complaining about low power. I'm seeking it.

Quote:
"...all the way to 7.62x39 performance..." The lack of 7.62x39 performance you mean. A Max load for a 125 out of a 24" .308 Win runs around 3000 or 3100 FPS. A 7.62 x 39 Max load runs 2200ish to 2,554 FPS, also out of a 24" barrel. The whole thing is about case capacity.
"All the way to 7.62x39 performance" means all the way DOWN to 7.62x39. Case capacity is not a concern here. The question is about designing a rifle that will reliably shoot a wide variety of power levels of ammunition. The whole goal is to make it reliably shoot weak ammunition.

Quote:
Anyway, if you go too low on velocity you will have unreliable functioning due to the load being too light.
That's the crux of the whole thing. Please read the details.

Quote:
And don't even think about loading below minimum loads. That can be dangerous.
I would never do that! Hodgdon provides data for H4895 in .308 Winchester. Their normal load data provides a min load of 42 gr for 2,796 fps in a 24" barrel. However, they ALSO say it's safe to load 60% of their max load for reduced power (using H4895 exclusively). That works out to 27.6 gr! I'm not sure what the velocity is but it's probably getting close to 7.62x39 levels, especially with a short barrel. So it is certainly possible to safely reload extremely weak .308 according to the books.
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Last edited by FizzyP; November 13, 2019 at 02:57 PM. Reason: left out barrel length
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Old November 15, 2019, 12:02 PM   #5
Bart B.
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Any time loads are reduced more than about 10% below maximum, the fired 308 case usually won't be fully expanded to fill the chamber. Case shoulder is set back a couple thousandths or more and maximum pressure ain't enough to push the back of the case against the bolt face.

Neck only resizing won't move the fired case shoulder forward. Subsequent neck only resizing soon ends up with incipient head separation cracks.

Full length resized such cases may not move the fired case shoulder forward enough.
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Old November 15, 2019, 01:14 PM   #6
FizzyP
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@Bart B. I follow your first sentence but after that you lose me. Do you really mean firing weak ammunition sets the shoulder back? So if I was to fire full power ammo, fitting the case to my chamber, and then reloaded something weak in it:
1) the shoulder would be set back
2) the case wouldn't be pushed against the bolt?

Can you elaborate a little? I don't understand.
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Old November 15, 2019, 07:52 PM   #7
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Reduced loads for .308 are relatively easy. Try the loads offered by Speer in their Reloading Manual #15. Whether you can get the action to cycle properly in an AR platform with the set-up you describe and the reduced loads, you would have to evaluate. Seems like it might be possible with a short gas system and adjustable block. It sounds a bit like making a complicated rifle where you have to know the settings for the varied ammo, and remember to make the change with ammo change. But, I suppose it might work.
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