Is loose fit of an AI AT barrel on purpose?

[ShtrRdy]

I have an AI AT in .308 that shoots great. I removed and then installed the barrel for grins one day. I was surprised that there was some definite slop in the threads between barrel and action. Have others noticed the same thing? Does AI do this on purpose?

 

[Supersubes]

They just work to a tolerance. If it was tight, like a br rifle, a grain of sand (since its a field rifle) would lock it up tight. Youre not giving up anything by having some clearance.

 

[hollowoutadime]

It's a clam shell thread design. The threads hinge open to allow removal and reinstall. Tightening the 4mm allen bolt firmly locks the barrel. This is why it return to zero on a reinstall.

 

[BurnOut]

They just work to a tolerance. If it was tight, like a br rifle, a grain of sand (since its a field rifle) would lock it up tight. Youre not giving up anything by having some clearance.

This. FWIW, I also run anti-seize on the breach threads.

 

[Servo93]

Because it uses a clamping design, the threads are purposely cut loose. The allows the barrel to be removed easily by hand and when the quick lock screw is tightened down all that slop is removed.

 

[scudzuki]

My AX308 is a 2013 (non-QC barrel) and the threads are not all that loose, but they're not as close a fit as when I barrel an action, which I try to get pretty close.
It makes no difference once I tighten it to 100 ft/lbs. I recently pulled my barrel to change the chamber lead-in on a lathe and after reinstalling it I completely forgot about it when I got to the range. Zero was off by a tenth of a mil in both axes.

I spun up a barrel for a friend's 2014 (QC) AT with the same thread pitch diameter as I made mine (reverse engineered from an AINA barrel) and the fit was about the same on his.

OP, not sure what you're calling slop between the threads. How much wiggle at the muzzle with the barrel loosened 1/4 turn?

 

[ShtrRdy]

My AX308 is a 2013 (non-QC barrel) and the threads are not all that loose, but they're not as close a fit as when I barrel an action, which I try to get pretty close.
It makes no difference once I tighten it to 100 ft/lbs. I recently pulled my barrel to change the chamber lead-in on a lathe and after reinstalling it I completely forgot about it when I got to the range. Zero was off by a tenth of a mil in both axes.

I spun up a barrel for a friend's 2014 (QC) AT with the same thread pitch diameter as I made mine (reverse engineered from an AINA barrel) and the fit was about the same on his.

OP, not sure what you're calling slop between the threads. How much wiggle at the muzzle with the barrel loosened 1/4 turn?

I take a look the next time I loosen the barrel. I want to shoot it the way it is for now.

Thanks for the replies.

 

[ShtrRdy]

When I removed the barrel a while ago I took a picture of the treads and clamping feature AI came up with. (they have a patent on it)

The threaded hole in the action has threads before and after the clamping portion which I would think would help align the barrel in the action.

 

[scudzuki]

The AI QC lock will push the barrel tenon radially into the fixed threads in the action directly opposite the slot between the 2 halves of the "lock".
It will also pull on the tenon axially (parallel to the axis of the bore) resulting in greater pressure between the tenon shoulder and the action face.
It's a fairly ideal solution as it will position the barrel repeatably in all 3 axes.

 

[ShtrRdy]

I finally loosened the barrel to see how much "wiggle" I get at the muzzle. It's between 1/8" to 1/4" of movement.

 

[MattyG]

Totally normal. The idea that guys need to have a super tight tolerance fit for a system to be accurate is a fallacy. The critical points are that the barrel shoulders square to the action and that the threads were cut true to the bore. Both of which you can guarantee AI has done.

 

[davidwiz]

Are you using a torque wrench when screwing the allen bolt back on? The Borka AT wrench works great and is pretty cheap (~$35).

 

[308pirate]

It's truly mind boggling that some amateur would question Accuracy International on rifle design.

A seriously WTF moment.

 

[delfuego]

It's truly mind boggling that some amateur would question Accuracy International on rifle design

Thanks for all your help sport. Please go back to the Bear Pit. The adults are speaking

 

[Mr. F]

When I removed the barrel a while ago I took a picture of the treads and clamping feature AI came up with. (they have a patent on it)

The threaded hole in the action has threads before and after the clamping portion which I would think would help align the barrel in the action.

Wow, what a clever design. Thanks for the Picture, speaks a thousand words.

 

[Steel head]

Wow, what a clever design. Thanks for the Picture, speaks a thousand words.

I was thinking the same thing.

 

[DAVETOOLEY]

Loose threads mean square shoulders. Square shoulders mean a secure-repeatable barrel receiver lock up.
Tight fitting threads add an unwanted variable to the overall personality of a system. Been there done that a long time ago.

 

[Ledzep]

Yeah there's little or no benefit to 1:1 thread fit. Galling, I guess, if you're into that... The tight thread fit feels cool when the barrel isn't torqued up and reinforces this mental condition that tight tolerances mean better, but.... Torque/preload and/or clamping (which applies preload) against a square shoulder is what counts. So long as the barrel doesn't move under load, it doesn't matter of it's a class 3 fit.

Obviously there's a limit; as an example there was a thread on here a few months ago where a guy was screwing a 1.0625x16 remage prefit barrel into a trued receiver (idk internal threads, but likely 1.075-1.085x16) and was having issues running out of windage adjustment, and being able to move groups around by pushing the barrel as he tightened it... But within normal spec range it doesn't matter.

 

[ShtrRdy]

Thanks for the explanations guys. I appreciate it.

-- Todd