Differences in CBTO length

[bdbrown66]

Hello, all. I have a question for you guys.

Background: I have 2 different 7-08 rifles. One is a Marlin XS7 that I bought used earlier this year. The other is a brand-new Ruger American. I just started measuring CBTO earlier this year (instead of COAL), and trying to find an optimum seating depth based on distance from the lands. This was my first opportunity to compare max CBTO (ie touching the lands) between 2 different rifles. What I found was that the measurements for the Ruger were .040-.060 shorter than for the Marlin.

And so, my question is this: Is this a reasonable variation that one might find between different rifles and manufacturers? Or is it indicative of the throat being shot out, etc? I know that some manufacturers (Weatherby, etc) are known for having long leades in their chambers, so is that part of the answer here?

Thanks in advance,
Brian

 

[flyingagg]

Not only it is normal for two rifles to have a difference, but that same rifle, the more you shoot, the longer that measurement will get.

 

[bdbrown66]

Not only it is normal for two rifles to have a difference, but that same rifle, the more you shoot, the longer that measurement will get.

Oh, without a doubt. My question was more like, "How much is too much?"

 

[ShadeTree]

Bd, it could of course be the Marlin has the throat shot out, but I doubt it. Eventually all rifles will get there with enough shooting, some way quicker than others. But in a 7-08 using normal loads and normal shooting habits, the optimum word is eventually. Takes some doing.

I have five 30-06's. Didn't check my new one yet, but I just got rid of one before this new one. I can't say the numbers for certain off the top of my head but I'm pretty sure there is at least .060 between the shortest one and the longest one when checking the same bullet. None of them have a worn throat problem that I'm aware of.

You will find most factory ammo is loaded pretty short for that reason. Can be a mile off the lands in some rifles, and running pretty close in some others.

 

[flyingagg]

Not only it is normal for two rifles to have a difference, but that same rifle, the more you shoot, the longer that measurement will get.

Oh, without a doubt. My question was more like, "How much is too much?"

I don't know how much is too much. I mostly load magnums, where jump is HUGE!. But I think different rifle/bullet combinations have a different sweet spot for jump and a "window" where it can be forgiving.
There is a serious of articles on bullet jump on PRB. I suggest reading them if you have not yet
https://precisionrifleblog.com/2020/04/ ... ment-tips/