Bore diameter VS jacketed bullet size

[Tripplebeards]

So I know the rule of thumb is to be .001” over bore diameter on a cast bullet. What about on a jacket bullet? It made me think yesterday when when slugging my rifle. My 35 whelen slugged at .3585” so I’m going to size my cast at .360”. Well what gets me thinking is jacketed ammo for my rifle has a diameter of .358”...smaller than my bore. I did try some hornady super performance ammo out of rifle last year and shot a 100 yard group that was probobably 1 1/4”. Not a great group for jacketed ammo but not horrible. My question is shouldn’t my bore diameter measurement be under .358” for .358” jacketed ammo use?

[Camper64]

Great question, I'm just an curious to the answer as well.

[Tripplebeards]

I went back-and-forth measuring my slug. It might’ve measured right at .385” as well which would be the exact same diameter. I was using the cheap harbor freight micrometer so I don’t trust it. I took tons of measurements on and off well recalibrating to zero to make sure it was reading correctly. I called Remington this morning since I bought the gun brand new last year just to pick there brain and ask them and they couldn’t give me an answer but said they’d be willing to have me send the gun in for them to test and measure. I don’t want to waste their time if that’s the correct diameter it should be from factory for jacketed bullet use. What was throwing me off is both my 35 Remington‘s measured at .357” and of course they all use the same caliber jacketed bullet.

Here’s my slug from yesterday that I measured from. The outside diameter is .358/.358.5” and I can see where the rifling did grab it.

https://i.imgur.com/biiB0n3.jpg

https://i.imgur.com/a7J8M08.jpg

[Valornor]

It kind of depends. SAAMI has a surprisingly large tolerance on bullet diameter for some calibers. It also has a tolerance for bore and groove diameter. So it’s up to the manufacturer to set their own spec as to what their bullet diameters will be.

These tolerances do stack so some bullets made at the smaller end of SAAMI spec, may shoot poorly in rifles with bored made at the larger end of SAAMI spec.

I have found most jacketed bullets to be pretty darn close to nominal bore diameter. Within a few tenths either way. If it was beneficial to be .001 over size or .001 undersized you would think you’d see it in SMK’s or Match Burners.




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[MostlyLeverGuns]

There is/was matches where all the competitors had ammunition supplied by the country where held. 7.62x51 NATO if I remember correctly. Such rifles were commonly bored quite tight, .305-.306 on the grooves to accomodate variations in ammunition, bullet diameter and maximize velocity. Lapua makes some 30 cal match bullets that are .3085 maybe .309 diameter. Powder gases do not affect copper jackets as much as they do lead so oversize fit does not play into accuracy as much. Most jacketed bullets are large enough to displace/bump up to fit rifling quite well. The toughness of jacketed bullets provides an advantage that the bullet caster can not hope to achieve with lead alloys, MUCH AS WE WISH TO. The benchresters also select hand-made bullets that have tiny 'bump' that helps with bullet fit. These variations in jacketed bullets are why some rifles like Hornady, some Speer, some Sierra. In the last 50 years, the increased precision of the jacketed bullet is probably the single most factor in the increases in modern accuracy compared to other improvements.

[Larry Gibson]

We regularly shoot undersized jacketed bullets in many of our rifles and sometimes we also shoot oversize jacketed bullets. As to undersized jacketed bullets there are numerous instances such as .264 bullets in 6.5 Swedes, .284 bullets in 7mm's (milsurp), .307 - .308 bullets in .3085 - .309 .30 cals, .310 - .312 bullets in many .31 cals with .314 - .316 bores and .323 bullets in S bore 8 mm's. Probably others out there also. Many of those mentioned will shoot jacketed bullets in the oversize bores quite well especially if the bullets are FB'd and have long bearing surfaces. Granted bullets of equal quality that are at or perhaps up to .001 over size will probably shoot better all other things being equal.

Softer cup and core jacketed bullets often obturate to seal the bore as do open based FMJs if not too under sized. I suppose we could get worried about gas cutting and such other but it really doesn't appear to be that much of a problem. Few casual shootings will ever wear/shoot out such a barrels. Only dedicated match shooters or "trigger freaks" actually wear barrels out from shooting them.

[Tripplebeards]

Thanks for the reply Larry and to the other posters, obviously my barrel is fine.