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Why are certain grain bullets recommended?

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Why are certain grain bullets recommended?

Old 10-07-2016, 08:07 PM
  #1  
Nontypical Buck
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Default Why are certain grain bullets recommended?

So a few weeks ago I was wondering what the rate of twist on my rifle is and couldn't find it anywhere for my cartridge, specifically. I went ahead and contacted Remington via e-mail and got this at the beginning of the response:

"The rate of twist for a .308 WIN 1:10 Faster Suggested Grain weights = 168,165,180..."

Why are they essentially recommending any standard factory .308 loading except the 150 grain?
I tend to shoot 150 grain soft points, since they're faster than 180s and the latter is unnecessary for shots within 200 yards. I consistently get MOA groups at 100 yards (with the same load, I used to get cloverleaf groups; I think it's because I need to do a thorough copper cleaning).

So I'm likely to keep doing what I'm doing, but I still want to understand conceptually what's going on. Why would I supposedly get better performance with a heavier load? I get that at longer distances higher ballistic coefficient can be a huge plus, especially on larger game, but that's strikes me as a general rule of thumb---not something that is self-evidently true specifically for 22" barrels with a 1:10 twist.
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Old 10-07-2016, 08:31 PM
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For a given cartridge, the bullet weight will dictate an approximate muzzle velocity it will produce. For that particular muzzle velocity, a given twist rate in the barrel will produce a specific rotational speed (RPM's) on the bullet. Each particular bullet weight (length, density, weight, and profile) will need a certain threshold rotational speed to stabilize.

For a 308win, a 180grn pill will run slower than a 150, AND it will be longer, so it'll need a faster barrel twist to stabilize.

In general, you have to try pretty hard to find factory ammo in 308win which isn't within proper specs for a factory twist rate barrel. 1:10" in a 308win will stabilize up to 200's most of the time, but nobody tends to shoot the 200's anyway.

Barrel length won't matter as much, in general, as the cartridge nor the twist rate, simply because there's not that much difference in performance (muzzle velocity with a given bullet weight) for a 20" barrel vs. a 24" barrel. In some cartridges, however, you'll see models with 16" barrels and other models with 26" barrels, where there might be 400fps or more difference between the two. In that case, a 16" barrel might need a 1:7" twist to stabilize a 77grn bullet, whereas a 1:9" twist in a 24" barrel might stabilize the 77's just fine (specifically use these as examples because I own rifles where this is true).

For lighter bullets, there's really no downside to running a twist which is too fast. They'll lose a tiny bit of potential velocity and run higher pressure than they could in a slower twist, but overall, you won't see any appreciable difference - as you're noting with your 150grn bullets.

Last edited by Nomercy448; 10-07-2016 at 08:36 PM.
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Old 10-07-2016, 09:10 PM
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Nomercy that was the best explanation on the topic I have ever heard. Thx!!
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Old 10-08-2016, 07:25 AM
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To hookeye's point - long (heavy) bullets do start to eat up case capacity. But it all depends what you want to do. I did load work up for a guy wanting to shoot 220's in a 308 several years ago for black bear. These were painfully slow and the trajectory reflected it, BUT, despite it's low velocity (would have to pull out the book to remember), I do recall there was still considerably greater momentum on the bullet at range than what I had on the 180grn load I had recommended to him. Momentum = penetration, so it had the advantage at long range, despite the trajectory.

I tend to recommend 165/168 or 180grn bullets for the 308win, across the board. 150's are fine, but at the lighter end of the spectrum for me. I shot 130-150 in 308win for many years, so maybe it's just age catching up with me, but I have seen enough to realize now I would much rather have a heavier for caliber, heavier for cartridge bullet than a light one --> WITHIN REASON! I'm not gonna start loading 200-220's in 308win any time soon.
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Old 10-08-2016, 10:38 AM
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I was having some fun trying to work up the best load for my .308 and kept coming back to 165 (168) pointed boat tail at around 2700 fps.

I eventually called steyr looking for a trigger recommendation and asked about their recommendation on ammo for accuracy. They said they had the best results using 167grn pointed boat tail at around 2750 fps.

My point is, sometimes it just works and much of the dialog is trying to explain why.
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Old 10-08-2016, 01:45 PM
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Thanks!
I carried 180 grains a few years ago when hunting mule deer and whitetails in the Nebraska panhandle, where shots can be anywhere between 75 and 350 yards; my idea at the time was simply that the (especially mule) deer in Nebraska are by far larger than those in TN, so more oomph cannot be a bad thing. It was only much later that I learned about the advantages of heavier bullets at longer ranges, despite the increase in bullet drop.
Should the weather warm a bit late in the season, I may decide to try my hand at black bear hunting late in the season in East TN, since those bears don't actually hibernate, but are often coaxed out of pseudo-hibernation on relatively nice days. If I do this, I'll definitely carry 180s---again using soft points, as 1) shots in the hills are not very far, so I don't forsee penetration being an issue, 2) 180-grain soft points are the 180s I have, and 3) I'm not probably going to get an expensive box of specialized bullets when the odds of seeing a bear in early January is probably remarkably low). If I actually become more serious about going after bear more regularly in the future I'll consider seeing how my rifle likes Nosler Partitions.
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