AXMC barrel timing

[NotMalware]

It's my understanding that when a gunsmith installs a barrel on an action they time the bend in the barrel to be centered at the top. How does it work with the AXMC or other similar system where they don't have the action to work with when they cut the barrel? Does the AXMC system throw away the barrel timing concept, or is the thread in the action start at the same place in all the actions?

 

[NotMalware]

I ask, because I was thinking about a proof research barrel for my AXMC, and I've heard that they have a comparably large runout. Is this a founded concern?

 

[Supersubes]

Yes, the barrel cannot be indexed without the receiver. I built a gauge for doing this, but need and old barrel off of the rifle with an index mark on it to index the new one.

Quality barrels are very straight, and you really don’t need to worry about it.

 

[NotMalware]

If it's not something to worry about then why did you build a gauge to deal with it?

 

[Supersubes]

The gauge was built for for measuring headspace primarily, but also works for indexing, and I index because I can.

Too many AI barrels out there shooting great for you to worry about imo.

 

[NotMalware]

That sound reasonable. Thanks for the perspective.

 

[OH58DViper]

My proof barrels for my AXMC are supposedly ready to ship. I’ll post pics and an accuracy review shortly after a trip to the range.

 

[bohem]

The barrel timing argument is largely pointless unless you're using a riflescope that's very short on elevation or windage travel. The majority of quality scopes made in the last 10 years make the whole barrel timing thing an ethereal and pointless argument.

 

[NotMalware]

The barrel timing argument is largely pointless unless you're using a riflescope that's very short on elevation or windage travel. The majority of quality scopes made in the last 10 years make the whole barrel timing thing an ethereal and pointless argument.

What's the argument?

 

[Mordamer]

What's the argument?

The argument is that you time the bend in the barrel bore to point straight up so that the bullet will impact high. The scope elevation then has to be adjusted to move the bullet impact down. This means that from your 100 yard zero you have more usable elevation in the scope. There really is no other reason to do it.

I have measured as much as 0.030" of bend in barrels from a very high end barrel maker. This is on the high end and most barrels would have less bend than this. This 0.030" over 26 inches of barrel equates to something like 1.1 mils of elevation difference for a 100 yard zero. Pretty insignificant with modern scopes that have oodles of internal adjustment and are most likely sitting on a base with 20 MOA or more of built in cant.

 

[NotMalware]

I've never heard that perspective on barrel run out. It does seem like that is certainly a concern with minimal significance with modern scopes. I have heard the discussion about POI shift due to thermal expansion of the barrel. The idea is that if a barrel were truly straight it would extend in length when the barrel warmed up, and the outer diameter would increase and the inner diameter of the bore would constrict as the steel of the barrel expanded in all directions. In a real barrel the curve will be exacerbated by thermal expansion causing the POI deviation to vary with temperature. The physics of that makes sense, but the extent to which that exists practically is something I don't know. I just don't have any experience with it. The excellent point made above about the number of Accuracy International rifles that shoot great without this timing seems to illustrate the practical scale of this affect.

 

[bohem]

I've never heard that perspective on barrel run out. It does seem like that is certainly a concern with minimal significance with modern scopes. I have heard the discussion about POI shift due to thermal expansion of the barrel. The idea is that if a barrel were truly straight it would extend in length when the barrel warmed up, and the outer diameter would increase and the inner diameter of the bore would constrict as the steel of the barrel expanded in all directions. In a real barrel the curve will be exacerbated by thermal expansion causing the POI deviation to vary with temperature. The physics of that makes sense, but the extent to which that exists practically is something I don't know. I just don't have any experience with it. The excellent point made above about the number of Accuracy International rifles that shoot great without this timing seems to illustrate the practical scale of this affect.

If you plot the curve generated for something in a nasty case, like what @Mordamer spoke of (0.030 on a 26" barrel) the runout due to a 100 degree temp change is highly insignificant. Less than can be corrected with the click of a scope at 100yd. The barrel changes in length by about 0.023" and the 0.030/26" becomes a 0.030027 runout arc.

That is 0.0972" of change in POI due to a uniform 100F temp change of a worst case 26" long barrel. Less than 1" of change at 1000yd.

For the typical runout I see in the barrels we cut here in volumes we are talking more along the lines of 3-5 thousandths in 26" IE 1/6th-1/10th of the number above.

Entirely insignificant

 

[NotMalware]

Thanks, Bohem. I can't say I have any concerns about barrel run out at this point. cheers.