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Old June 14, 2012, 03:14 PM   #1
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undersized bullet: what to expect?

Caliber: .327 Federal Magnum
Handgun: Ruger GP-100, 4-inch

I've been loading this round for a bit more than 4 years. I've been using the Hornady 85 and 100gr XTP bullets for most of my ammo, though I have dabbled in .32 ACP bullets, namely the 71gr FMJ and 71gr Berry's Plated Round Nose slugs.

.32 Auto, .32 H&R Mag and .327 Federal Mag is designed around the use of .312" diameter bullets.

I also load .30 Carbine for use in a Ruger Blackhawk. Though I've dabbled with Speer bullets (100gr "Plinker" and 110gr FMJ), I pretty much load a lot of the Berry's 110gr Plated. This has been a terrific bullet for my needs and Berry's rates it for carbine use at speeds up to 1,900 FPS, which I don't even approach in a handgun load.

.30 Carbine bullets are .308" in diameter.

I have taken component slugs and tried them in the barrel (at each end) and in the cylinder. You really can't see a difference between the .308" and .312" slugs in this manner. Neither will enter the muzzle, neither will go far in to the forcing cone area before meeting resistance and though both are loose at the breech end of the cylinder, both will fit through the forward end of the cylinder, but neither with any manner of play.

I've decided that I'm going to try some of these bullets in .327 Federal. In a sized and flared case, they have some mouth tension. I haven't run them in dies yet nor attempted a crimp of any sort, so not sure what I'll see there. (my experiment may end at this point if I can't reliably get them to hold their place in a loaded round)

Assuming that I can safely get these slugs to fit & stay in the there anything we can expect or presume when it comes to loading jacketed or plated slugs that are three thousandths smaller than spec?
Attention Brass rats and other reloaders: I really need .327 Federal Magnum brass, no lot size too small. Tell me what caliber you need and I'll see what I have to swap. PM me and we'll discuss.
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Old June 14, 2012, 04:06 PM   #2
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I would assume accuracy will be lousy along with heavy jacket fouling in the bore.

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Old June 14, 2012, 04:09 PM   #3
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Abysmal accuracy is what to expect. I mean awful. Even .001" too small will shotgun pattern. Speer or Hornady makes the bullet that will actually work. I'd look around a little more.
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Old June 14, 2012, 04:18 PM   #4
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" there anything we can expect or presume when it comes to loading jacketed or plated slugs that are three thousandths smaller than spec? "

No, not with any certainty. If your bullets are heavy (for caliber) and your charges are hot they will likely up-set enough to fully obturate the bore. If the bullets are light and/or the charge is light obturation is much less likely and the comments above will apply.
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Old June 14, 2012, 05:20 PM   #5
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You honestly won't know until you try. Unless you have slugged the bore of your individual gun it is impossible to say exactly how undersized the bullets will be. People tend to assume that their bore is what it is supposed to be but I guaranty you there are people on this very forum shooting .355 jacketed bullets out of there 9mm's that are in actuality .004 too small and getting just fine accuracy. Ignorance is bliss.
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Old June 14, 2012, 06:57 PM   #6
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Did I miss something? When did you get a .327 GP100?

As others have said... there's only one way to find out.
We can theorize all day about fouling, accuracy, gas cutting, throat erosion, and obturation; but you'll never know for sure, unless you try it.

Cornbush, for example, has a 7.7x58mm Type 99 Arisaka that was given to him by our grandfather. While our grandfather was secretly building the Arisaka for him, he also worked up a couple loads with .308" bullets, that were shooting sub-MoA at 100 yards.

After we experimented with quite a few .311", .312", and .313" bullets, with mediocre results, Cornbush slugged the bore. It was .314".

...A .314" bore that doesn't like .32 caliber projectiles, but shoots .308" like crazy. And, it doesn't cause excessive fouling.

I've had mixed results with my M38 Mosin. My initial tests with .308" and .310" bullets were rather abysmal (minute-of-mountain-side). But some more recent tests have shown decent accuracy potential (still developing the loads).

I think a big part of it with these rifles may be the deep grooves, though. My M38 slugs at .301" bore / .315" grooves. Cornbush's Arisaka (I believe) slugs at .302" bore / .314" grooves. The tall lands still engrave the bullet very well, and might be causing enough jacket deformation (not really obturation) to seal the grooves. We've never been able to recover one in good enough condition to take accurate enough measurements, to know for sure.

I suspect that trying the same thing in a firearm with shallow rifling would result in terrible accuracy. My .444 Marlin Handi-Rifle, for example, does not have a true micro-groove barrel, but it slugs at .428" bore / .432" grooves. Shooting even .429" bullets leaves obvious signs of the bullet slipping on and skipping over the lands. (though, I've never done formal accuracy testing, to see just how much it is affected)

The grooves in the GP100 should be deep enough to at least make it worth trying.

I've really been wanting to get out with my own .327 GP100 lately. Maybe I should go fondle it for a while...
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