Gunsmithing Projectile diameter vs nominal bore diameter... how tight is too tight?

[BurnOut]

In my 450 Bushmaster/45 Raptor bolt gun saga, I have decided that there aren't enough .452 (450 BM/45R) diameter rifle projectiles to make me happy, so I'm playing with swaging down some copper jacketed .458 projectiles; specifically, some 325 gr Hornady FTXs. After swaging them down in two steps (.454, then .451), my final results are averaging .45215, with a low of .45180 and a high of .45250; the question is, will it be safe to run these through a bore with a nominal diameter of .452?

 

[Mordamer]

My opinion is yes. Back when 300 Blackout was first coming into existence I shot a bunch of the .310" diameter 123 grain SST's through my .308" diameter bore. They were super cheap compared to similar .308 bullets. I used a starting load and never had a single issue. Make sure your first loads are well under max. I think you'll be fine.

Also, your measurement significant figures are a little over the top.

 

[Ledzep]

A few thou usually doesn't mean much. Increased fouling and heat. I've shot .312 bullets in a .308 and never noticed a difference, I'm sure pressure was slightly higher but it wasn't drastic. There's lots of folks (knowing or not I can't know) shooting .358" bullets in the .355 groove dia. of their .350 legend. Lots of Obermyer and other barrels would taper down to .307 at the muzzle end.

That said, there's Savage barrels that are rough as shit and .0005-.0020" undersized in bore and groove that will foul up, gain 200fps, and blow primers in 50-100 rounds...

 

[BurnOut]

Also, your measurement significant figures are a little over the top.

I know that it's a bit ridiculous to go into the sub-thou range on measurements, but I wanted to convey the OD of the projectiles as accurately as possible.

In any event, the bore in question is a factory Ruger piece; I just wanted to make sure that I wasn't setting myself up for a kaboom.

I appreciate the replies, gentlemen.

 

[dustingaunder]

The point with the significant figures is that it is unlikely you are using a tool that is even capable of measuring a one hundred thousandth. A 1/10 vernier on a good micrometer I wouldn’t even mention most of the time. More decimal places isn’t more accurate. It’s more precise.

 

[BurnOut]

The point with the significant figures is that it is unlikely you are using a tool that is even capable of measuring a one hundred thousandth. A 1/10 vernier on a good micrometer I wouldn’t even mention most of the time. More decimal places isn’t more accurate. It’s more precise.

If it makes you feel any better, I was getting those numbers with a Mitutoyo QuantuMike rather than a set of calipers. Regarding your point about accuracy vs. precision, point taken; I would have to be certain of my instrument's calibration in order to have any confidence in its accuracy.