I’m curious to your barrel length suggestion. Are you saying the shorter the barrel, the less friction on the bullet ? The reason I ask is I. Shooting a 6.8 twist finished at 33”. I’ve mainly been shooting 80smk at 300 but have shot just enough 85.5 to make sure the load is going to shoot. I’m not sure of velocity but I would hate to travel to a match just to blow up bullets.
Thanks,
Blake
The shorter the barrel, the less friction heat will be generated, with all else such as bullet choice, velocity, twist rate, and bore/groove configuration remaining the same. That is simple physics. However, stating that some specific
barrel length will be associated with jacket failure cannot realistically be done with certainty because other contributing factors such as twist rate, bore/groove configuration, bullet velocity, bullet bearing surface length factor into the equation. Nonetheless, I think it would be fairly safe to state that if all else is equal, a 26", or 28" barrel isn't going to put the same amount of heat and/or stress into the bullet that a 30" barrel would. Exactly where in terms of barrel length the specific cutoff point for jacket failure might be is almost an unanswerable question due to the involvement of the other contributing factors.
In terms of realistic expectations, a 33" barrel is pretty long and would potentially put a checkmark in the "unfavorable" column, in terms of increrasing the potential for jacket failure. Likewise, a twist rate faster than 7.0
might be considered "unfavorable" in combination with certain other features. For example, you didn't mention whether you were using a 0.218" or 0.219" bore barrel. A 0.219" bore would be a checkmark under "favorable", whereas a 0.218" bore might be considered "unfavorable", and would combine to place the 6.8-twist rate in the "unfavorable" column. In contrast, the bearing surface lengths of the bullets you're using range from just a tiny bit shorter (85.5s) to much shorter (80 SMKs) than the Berger 90 VLD, a bullet known to suffer jacket failures under conditions where too many of the "unfavorable" factors combine. The shorter bearing surface lengths would be considered a "positive". You also didn't mention the velocities you are obtaining with these bullets, which can also be a contributing factor if very high, or a positive factor if moderate.
The bottom line is that there is no realistic way to add up all the potential positives and negatives and come up with any "quantifiable" answer. Bullet manufacturers will often state that it is unwise to push bullets past 300K RPM, but that is also a rather nebulous term when used by itself. At some point, you just have to shoot the load and see what happens. I am all too aware of the frustration involved in having a jacket failure occur during a match, but unless every single indicator is pointing toward an extremely high likelihood of jacket failure, you can't know with certainty that it's coming. The more you test and/or practice with the load, the more confidence you may be able to develop that nothing bad is going to happen.
FWIW - because I know with certainty that several contributing factors are going against me in terms of potential jacket failure with one of my F-TR rifles, I worked up a method to try and lessen the odds that it would happen during a match. Because the jacket failures I have had in the past usually occured once I had put at least 40-50 rounds down the barrel during practice or at a match, my idea was simply to remove a fair bit of the carbon fouling after each of the first two matches of the day (three matches per day total). I run three or four patches wetted with Kroil while the barrel is still hot, followed by three or four dry patches. This removes a substantial amount of carbon fouling, but will not "un-season" the barrel by removing any copper. I tested this approach several times during practice to convince myself the first sighter after this cleaning process (i.e. for the next match) isn't totally wasted. What I have found is that it might be just a tick low, but not so low as to drop out of the 10-ring. Thus, I am not wasting a sighter by cleaning the barrel in this manner. More importantly, I have never once lost a jacket in match after I instituted this cleaning procedure, even in a tight bore, fast twist barrel that had been known to cause jacket failures. So it works. Yes, it's a PITA to have to do this at a match. Nonetheless, it's not nearly as painful as losing a bullet (and the match) due to a jacket failure. I am not suggesting that you need to adopt this approach, merely offering it as an option in the event you have stronger reason to believe your load is at high risk of causing jacket failure. At this point, I don't think there is sufficient evidence to point one way or the other. So I would just try the load out first, and see what happens. It may be that you need to be concerned about it. But it may be that it's simply not an issue at all in your setup.
