AI MXMC 338 LM freebore?

[bunny too]

I am new to reloading and was attempting to measure my freebore in my new AI MXMC 338 LM rifle. I am using the hornady O. A. L gauge method. When comparing my measurement (3.0260 base to ogive for a cartridge seated with a bullet touching the lands) to a cartridge I can reliably load in my magazine (2.9720 base to ogive) gives me a freebore (or "jump" not sure which is appropriate term) of .0540 by my calculations is this possible? I realize it is a military rifle with (probably) an oversized chamber to accommodate debris and fouling but this seems terrible excessive. I am achieving quarter to half minute of angle groups so accuracy is not an issue. Everything I have read stated that a bullet seated two one-thousands off the lands was desirable for bolt guns. I am confused. The rifle has two hundred rounds through it. Any help shedding light or clarifying the issue would be greatly appreciated.
Thank you

 

[bunny too]

**I was using Hornady 185 gr ELD bullets to test.

 

[spife7980]

The term is jump.
Freebore is the section of the barrel in front of the case with the rifling removed. Jump is the difference between the end of the freebore/start of the lease where the bullet hits and it’s location when seated in the case.

.054 is fine.

 

[6brshooter]

Results on target is what matters, the rest is just noise.

 

[CoryT]

For further info, I'll point to this well written series of articles.

Bullet Jump Research

 

[bunny too]

For further info, I'll point to this well written series of articles.

Bullet Jump Research

Thank you!

 

[6brshooter]

I forgot about that, pretty good info. Berger also talks about testing their vlds at .020 jam & .020, .040, .060, .080, .100, and .120 jump. The whole "vlds need to be jammed to shoot good" is a bs myth. Every barrel is different, throw in leade angle and other minute chamber differences, treat each accordingly.

 

[Scott Seigmund]

I realize it is a military rifle with (probably) an oversized chamber to accommodate debris and fouling but this seems terrible excessive. I am achieving quarter to half minute of angle groups so accuracy is not an issue.

Everything I have read stated that a bullet seated two one-thousands off the lands was desirable for bolt guns. I am confused. The rifle has two hundred rounds through it.

Thank you

Oversized chamber - NOT! That term generally infers a big diameter, sloppy chamber in a rifle that isn't expected to have high precision. The chamber in the AI 338 Lapua rifles conforms to the CIP specification WITH THE EXCEPTION of the freebore length. We extended the freebore significantly a few years ago following a large scale test of production rifles. You can read something about that here. https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/ (about half way down the page)

I'm not surprised at the results you are getting with handloads.

As for "one-thousands off the lands", forget that garbage. It's a hold over from a bygone era of low tech bullets and sloppy, oversized chambers. It's a way to get better alignment with the bore in big "factory" chambers. Partial neck sizing is also is from that era for the same reasons.

I'm glad to help.

-Scott

 

[bunny too]

Oversized chamber - NOT! That term generally infers a big diameter, sloppy chamber in a rifle that isn't expected to have high precision. The chamber in the AI 338 Lapua rifles conforms to the CIP specification WITH THE EXCEPTION of the freebore length. We extended the freebore significantly a few years ago following a large scale test of production rifles. You can read something about that here. https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/ (about half way down the page)

I'm not surprised at the results you are getting with handloads.

As for "one-thousands off the lands", forget that garbage. It's a hold over from a bygone era of low tech bullets and sloppy, oversized chambers. It's a way to get better alignment with the bore in big "factory" chambers. Partial neck sizing is also is from that era for the same reasons.

I'm glad to help.

-Scott

I am actually getting .5 MOA with the least expensive factory ammo I could find, Sellor & Bellot 300 grain.