How to Calculate freebore???

[Fordnutter]

I'm hoping someone here can help me. I'm wanting to get a reamer made from PTG for my 300 WSM.... problem is I don't know what freebore I require. I measured up a variety of bullets that I wan't to shoot & found that if I seat the boat tail junction of a 190gr Berger VLD just about the case neck / shoulder junction this will then let me shoot 190gr all the way down to the 165-168gr & even some of the longer bearing surfaced 150gr projectiles. As I have a 1-12" kreiger sitting here I figure this should be a great compromise to allow a large variety of bullet combinations. Coincidently the extreme base of the 168gr Amax lines up with the neck / shoulder junction point so I stated this with the overall length to PTG with my reamer order thinking that this should give them something to work from with what I'm trying to achieve. I received an email back asking what freebore I want, send a dummy round. So I reply that could be problematic as I live in New Zealand so how do I actually measure this. Response back from PTG was didn't my gunsmith have an idea ??? (in my case he hasn't built a 300 WSM before).

So how on earth do I calculate what freebore I want by measuring my dummy round that I have? I was thinking that maybe something like a .3075" gauge slipped over the end of the projectile & measure from there to the case mouth....but what about the 20 thou+ allowance from end of case neck to start of throat etc & how do I allow for that & not cock this up?

Help please.

 

[jscandale]

Load a dummy round to the length that you would like it to be. Get a 1" micrometer (not a vernier calipler), a vernier caliper, and a shaprie marker. Have them ready when you call to order your reamer from Dave Kiff. He will walk you hrrough it very easily.

JS

 

[jsn]

You might want to post this on the Competition Forum. Someone has probably done what you're trying to do and may have a reamer already spec'd. Also, I have seen JGS reamer prints where they have notes regarding which bullets the throat is cut for, and where the throat datum line is from the bolt face.

I'd send an email to JGS seeing if someone has already done this and if you can have the info.

 

[Rustystud]

The term free bore is used loosely used, kind a like folks use headspace incorrectly. Look at a PT&G reamer print you will see two measurements in front of the case. One is called the "Lead" and the other is the "Throat). The combination of these measurements what is commonly called the "free bore". The true meaning of "freebore" is the combination of the lead and throat in front of a seated bullet. That portion of the chamber that the loaded round can jump before encountering the lands. I suggest that one read Wayne's definitions in the 4th edition of Frank delap's Bolt Action Rifle.
Yes, I know there will be some who disagree with the aforementioned description. They want agree on head spacing either.
Nat Lambeth

 

[Dave L Kiff]

I have a dead on way to figure out the free bore on your dummy round with out sending it to me . Call and I will teach you. 541-826-9244 . Thanks Dave

 

[X Ring Accuracy]

I FULLY ecpect Dave to break out here in this forum in a tutorial on that subject. If he lets the forum down, I will begin a long campaign by calling him everyday shaming him for his lack of compassion. Good fun! Give him a call as he can explain it better than I can as he is the master.