Bullet to Groove Diameter

Discussion of BPCR loading techniques, SAFETY, Case Cleaning and Prep, Indexing, BP Choice, Primers, Wads, Compression, Drop Tubes, Vibration, Load Testing, etc.
SgtDog0311
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Bullet to Groove Diameter

Post by SgtDog0311 » Sun Jan 07, 2018 1:41 pm

Just curious about opinions of those shooting grease groove bullets and BP.

Do the majority order molds/use bullets that are a thousands oversize or do you prefer a groove size bullet?

martinibelgian
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Re: Bullet to Groove Diameter

Post by martinibelgian » Sun Jan 07, 2018 2:33 pm

I don't think there is a single right reply. Normally, I would go for whatever fits the fired case mouth/throat of the chamber. BUT I'm also currenntly shooting a .458 groove-dia. PP bullet in a very sloppy chamber, down a .462 bore with excellent accuracy. I use fireformed brass, the bullet will fall out as it is not at all retained - but still it shoots very well with BP. And of course, you can also shove a bore-diameter PP bullet in you barrel and get very good accuracy with a .451 bullet in a .458 barrel - again with BP.
To summarize, you'll have much more important variables to contend with than a mere .001 bullet diameter difference...

SgtDog0311
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Re: Bullet to Groove Diameter

Post by SgtDog0311 » Sun Jan 07, 2018 5:00 pm

martinibelgia, appreciate the response. I am indeed contending with the other variables. Interesting to note I just had a similar report from an experienced PP shooter with a bullet patched smaller than bore diameter that performed very well and it surprised me. I had thought before his report that a bore diameter or even a thou or so over was almost mandatory.

But I specified grease groove because I'm drawing up a mold at the moment and considering a .459 or .458. So my curiosity was along those lines. I'm thinking the preponderance of 'grease groove' BPCR shooters aim for slightly over but was wondering if I might flush out some other opinions based on practice when a fellah had his choice.

Brent
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Re: Bullet to Groove Diameter

Post by Brent » Sun Jan 07, 2018 7:47 pm

shooting a bullet that is smaller than bore diameter can certainly be done and it might be fairly accurate, depending on how small it is. But, as a general rule, bullets that are snug in the bore when chambered (that is there is a slight resistance when they are chambered) will shoot better than those that just drop in. In no case will "drop in" bullet shoot better than a the same bullet that is a snug fit. You want a good fit simply to center the bullet as precisely and accurately as possible. A "drop-in" bullet will not do this better than a snug-fit bullet.

I don't know who you were talking to (though I could guess), but consider that the folks that are actually shooting the best scores with paper patches are not using undersized bullets. NONE of them. And it's not because they haven't shot them.
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Re: Bullet to Groove Diameter

Post by Ranch 13 » Sun Jan 07, 2018 7:58 pm

With the grease groove bullets groove diameter to .001 over is plenty. However if the rifles chamber is a bit on the generous side, then you'll need to find something that will fit the chamber and not give you problems from gas cutting the bearing surface of the bullet,but not be so large as to cause finning of the base as the bullet travels down the tube. Another thing to be concerned about with a grease groove is to be sure the nose diameter doesn't exceed the diameter of the bore. If you have much of the nose that is over bore diameter you'll get leading due to the lack of lube on that nose you'll get some tinning on the top of the lands.

Kurt
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Re: Bullet to Groove Diameter

Post by Kurt » Sun Jan 07, 2018 9:10 pm

For a GG bullet I opt to go a thousand over bore this way you have the option to size it a thousand down if need be for accuracy it the rifle shoots better at the lesser diameter. You can always reduce it but it's harder to expand it with out a swage. Also a potential problem could come if the shank is to tight for the bore/groove pushing to much lead back past the base. But at groove diameter with blk the bullet will seal tight when the charge goes off even if it's a thousand under groove diameter. It's the ogive shape I would be concerned about how it fits the throat. And another thing to think about is the use of the bullet, hunting or match. For match it's best to have the ogive to make contact with the throat and slide in the bore with a snug contact but not so tight that you have to double thumb the case head to get it seated. Cleaning or blow tubing between shots for match a snug is a plus. But to much bore riding portion of the ogive can also cause a problem leaving lead smears.
For hunting that is a different story. You want a ogive that will load for a follow up shot. This is also a concern for the PP for match or hunting.
It's best to have two moulds. One for match and the other for hunting or just busting rocks :D
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Brent
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Re: Bullet to Groove Diameter

Post by Brent » Sun Jan 07, 2018 9:20 pm

Kurt wroteCOLON
Sun Jan 07, 2018 9:10 pm
For hunting that is a different story. You want a ogive that will load for a follow up shot. This is also a concern for the PP for match or hunting.
It's best to have two moulds. One for match and the other for hunting or just busting rocks :D
Truth! :)
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bruce moulds
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Re: Bullet to Groove Diameter

Post by bruce moulds » Mon Jan 08, 2018 1:14 am

sgtdog,
your question needs to address what happens to a bullet in a case when a charge of black powder goes off behind it.
what happens is that the case instantly obturates against the chamber wall, and as it does sothe bullet obturates against it, bumping up to fill any available space.
alloy hardness can control the rate and level of this bullet bumpup to some degree, too soft an alloy giving the bullet great friction against the case wall, and too hard offering little or no bumpup and therefore no seal and flame cutting.
these alloys can be between plain lead and lyman no 2.
as you can see, worrying about the cast bullet diameter matching the groove diameter of your barrel is a futile exercise.
the best diameter bullet is one that will slide into a min twice fired case, and can be pulled out again with a pop, by hand.
this bullet will then be minimally upset in bumpup, and therefore offer best accuracy mostly.
for hunting, the same bullet diameter can be loaded into a sized case with 0.001" neck tension.
as the bullet travels forward and enters the barrel it is swaged to fit whatever space is there.
this is where loose chambers cause excess sizing down and reduced accuracy.
keep safe,
bruce.

SgtDog0311
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Re: Bullet to Groove Diameter

Post by SgtDog0311 » Mon Jan 08, 2018 8:13 am

Thanks for the replies fellas. That question of what happens when the charge goes off is one reason I was going back and forth between .458 and .459 (for the record, I never entertained undersized bullets and no one was lobbying me to do so). The "obturation" seems to almost negate the need for that. For bullets used with Smokeless I've always favored at least .001 oversized but just wondered what most BPCR guys favored when ordering or cutting a mold. The logic of continuing to go a little oversized in order to retain the availability of sizing to exact groove is the most compelling for keeping the shank .001 over groove size. Appreciate the responses.
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Re: Bullet to Groove Diameter

Post by Ranch 13 » Mon Jan 08, 2018 9:06 am

John what sort of superduty bullet are you comtemplating?

SgtDog0311
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Re: Bullet to Groove Diameter

Post by SgtDog0311 » Mon Jan 08, 2018 12:42 pm

Don, I'm pretty set on reducing the weight on a couple of old classics. I have the Ideal 457132 Postell and the Lyman 457121 on my screen trying to reduce the weight to around 475gr (about 88% of the original 535 weight) but trying to still remain as close to the original radius and nose profile as I whittle them down. I work in Tom Meyers TNT software and as I make changes I keep an eye on the balance between Center of Gravity and Center of Pressure to keep the Form Factor %ages near the original as well as the BC. As you scale one down you can't help but lose some in the last two categories but you can watch them change as you peck away and back up or press forward as you see fit.

Pretty certain I'll eventually have the Saeco 645 I've seen you recommend. I'm one of those who butts their head against the tree trunk a while chasing an idea. The Saeco will likely be #3 on my list after my forehead gets bloody enough.

Shooting a Ballard, so being a little lighter than the Sharps, its one reason I'm scaling down.

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Re: Bullet to Groove Diameter

Post by Ranch 13 » Mon Jan 08, 2018 3:10 pm

John, Steve Brooks has the original Ideal profile dialed in pretty well. I have two from him for the 44's.
That Saeco 645 sure treats our 45-70's well.

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