Calculating Freebore

Jud96

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Joined
Jun 30, 2013
Messages
3,053
Location
Michigan
For awhile now I have been trying to mathematically figure out a way to determine optimal freebore for a given cartridge and bullet. Obviously the easy way is to send a dummy round to your smith or the reamer maker and be done. I however would like to figure out a way to accurately determine freebore length without needing anything more than numbers.

Berger has a really nice chart that lists their bullet specs and finding cartridge dimensions is easy online or in a loading manual. My original thinking was to start with the bearing surface length of the bullet. Let’s pretend here and say the bearing surface is 0.500”. Then I would look at the case neck length, let’s say it’s 0.300”. Now let’s say the lead angle is 1.5 degrees and it’s .020” long. I would simply add the lead angle to the neck length and then subtract that from the bearing surface length to get the freebore length needed to have the bullet seat right at the neck/shoulder junction. That number in this scenario would be 0.500 - 0.320 = 0.180”. So the freebore needed would be 0.180” plus say .030 to keep the bearing surface above the neck shoulder junction when seated off the lands. So my previous method would determine 0.210” of freebore would be optimal in this made up combination. However, I have found that often times my freebore calculation ends up being too long by say .050 or so. This can be a problem when COAL is important or if you’re trying to shoot multiple bullets in one chamber design.

My question is, how can I accurately determine freebore length within +-0.020 for any bullet and cartridge combo? It seems that I’m missing something and/or doing my calculations wrong. If anyone has any insight or knows how to do what I’m trying to do I would love to hear your thoughts and what you do. Thank you!
 

MagnumManiac

Well-Known Member
Joined
Feb 25, 2008
Messages
3,317
You’re forgetting that there is a buffer zone at the end of the chamber for brass growth in the neck. You will need to look at the reamer print for each cartridge.
Also, the angle to the rifling is not always the same. Reamer prints are the only way to know this.

Cheers.
 

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