.218 bore question

[SurgeonCO]

So I’ve got a Bartlein 224 barrel, 7 twist and .218 bore, modB. Got a good deal on it. I’m curious on everyone’s input on this thing grenading bullets over 3000fps. I’d like to do a 22GT. I’ve got one chambered in a proof, don’t remember the twist. Shooting 90smk 3060. This Bartlein we be well over 300k rpm’s at 3000 fps.

 

[Ned Ludd]

I can cause jacket failures fairly easily with a .223 Rem using a 30" Bartlein 5R 0.218"/0.224" 7-twist barrel. You can thus imagine what might happen with a larger .22 cartridge capable of much greater velocity with heavy bullets .

However, there are some caveats. With a 0.218" bore barrel, there are several other considerations with respect to how likely it will be to cause jacket failures. It is important to note that jacket failures are first and foremost a product of excessive friction. Barrels of 28-30" or longer seem to be where the problem really starts. Pushing heavy .224" bullets that have extremely long bearing surfaces such as the 88 ELDM and 90 VLD hard may be problematic in 30"+ barrels. In this scenario, long 0.218" bore barrels with twist rates faster than 7.0 can become the "straw that broke the camel's back" in terms of promoting jacket failure. LIkewise, generating RPMs close to (or over) 300K may be a problem. Long strings of fire and/or shooting on very hot days can exceed the limits. The longer the bearing surface of the bullet, the greater the risk of failure. Thinner bullet jackets may also be a contributing factor. But keep in mind that these are really only additive factors that collectively push the system beyond acceptable tolerances. The real key is friction, and I say this because some of the other factors such as twist rates faster than 7.0 seem to completely disappear when going to a slightly larger 0.219" bore barrel, even at barrel lengths of 30" to 32", all else being equal. For example, running 88/90/95 heavies in a 6.5- or even 6.0-twist 30" barrel does not seem to be any problem at all if it has a 0.219" bore.

So if you can stick to a barrel length of something like 26" or less that will still allow you to generate acceptable velocity, you may never notice a problem with the 0.218" bore. Using an 80-something grain bullet rather than those in the 88/90/95 gr weight range will likely elicit fewer problems. Compared to some other bullets of comparable weight, the Sierra 90 gr Matchkings seem to have about the toughest jackets out there. That doesn't mean they can't also be made to fail, but they will likely still hold together at the point some other brands will start to fail. Although you stated the twist rate of the barrel you already have is 7.0, reducing the twist rate for bullet that will be pushed close to the 3000 fps velocity range or beyond will likely also be beneficial with a 0.218" bore. As I mentioned previously, the best solution for the bulk of these issues is simply to go with a 0.219" bore barrel and the contributing factors seem to largely go away. However, there are still a few things you can do to help mitigate jacket failures with a 0.218" bore barrel as I noted such as using shorter bullets, a shorter barrel, etc. The best you can do is try some of these approaches and hope the jackets don't fail. If it still happens, it may be necessary to reduce the load until the issue stops, or replace the barrel.