Measuring the 450 Bushmaster Freebore.

[Texas Sheepdawg]

0.240"?
Give me a hand here. Is this excessive freebore to the lands going to be a detrement to accuracy?
Or should I consider that a very cooperative chamber?

[Hoot]

0.240"?
Give me a hand here. Is this excessive freebore to the lands going to be a detrement to accuracy?
...snip...

Sure looks like it.



Hoot

[Texas Sheepdawg]

LOL! Yeah. I was worried about how much freebore I had. Looks like I've got plenty.

[tbirdman74]

I seem to recall Hoot posted a pic of the same thing, bullet in backwards, and his was similar to yours. I, on the other hand had the exact opposite. Zero free bore. As near as I could measure with the rudimentary tools I had on hand, I had a .4495-.4500 leader in my chamber. Still waiting for the smithy to get the reamers in to clean it up.

[commander faschisto]

I would guess the WildCatter could chime in here with the design dimension for the leade??

Earth to WC....

[Texas Sheepdawg]

I've emailed him on several different occasions about various things but he hasn't answered. He's probably sneaking up on his next plate of s'mores.

[Hoot]

Here's a Link to the SAAMI chamber drawing. Looks like .2 inches

Hoot

[wildcatter]

Here's a Link to the SAAMI chamber drawing. Looks like .2 inches

Hoot

That's correct, the print shows .200" free-bore.

TSD, the reason you're getting .240" is most likely the radius of the heal of the bullet letting it go further into the chamber.

Hoot has written several good methods for determining oal.

I usually, assuming you don't have a Gage, use a sharpie, mark up the ojive, on a dummy round and chamber the weapon. If you get marks from the lands, on the bullet, keep seating them deeper, into the case and then retry, until you no longer see the marks. But, if semi-auto work is the goal, you are limited by the mag length. So, I'd start out at the max mag oal and work backward.

This is but one method, of many..

..t

[Texas Sheepdawg]

Using the Hornady gauge, a 230 grain FMJ round or flat point just barely stays inside case when fully pushed to the lands. By barely, I'm talking less than approx 0.150".

[Hoot]

Using the Hornady gauge, a 230 grain FMJ round or flat point just barely stays inside case when fully pushed to the lands. By barely, I'm talking less than approx 0.150".

Cup and core solids are short for their weight due to their density. Hollow points are longer since some of their insides are air. Copper hollow points even more so as copper is less dense than lead. The Barnes 160gr TAC-XP is the same length as your 230 FMJ. Both the 185 and 200gr cup and core FMJ's at the lands in my rifle are outside the case.

That generous freebore is the price we pay for being able to load the real heavy bullets, especially ones with a relatively blunt tip. Given my predilection for lighter bullets in the 450b, if I were to have a new barrel made, I would probably have a slightly shorter freebore since I works all my loads up starting down a few grains from optimum. Long jump loads still can be made to shoot accurate if you do you best maintaining concentricity and uniformity in your bullet seating and crimp.

Hoot