Multi-Caliber Rifles... What do you expect?

[Rasyad]

In general, when swapping barrels what kind of offset of the zero are you experiencing between calibers, and or barrel length? I am getting a 7.25" offset at 100 yards. Is this typical?

I have a couple new barrels on my gas gun, a 13.5" 308 with a pinned and welded muzzle brake shooting 175 gr FGMM, and a 20" 6.5 CM shooting Hornady 140 gr ELD Match. I have had the rifle to the range a few times and the results are consistent.


Thanks in advance for you help in understanding if I have a problem with my expectations or with my equipment.

Rasyad

 

[newguy2k3v2]

I have a few different ones and the largest shift I get is about 1 mil but .5 is pretty normal. 7" seems like a lot but if it's repeatable I would think it would be annoying but not the end of the world.

 

[Rasyad]

Thanks for that data point, I would have been happy with 1 mil but this is twice that. As well it seems odd that the flatter shooting 6.5 cm impacts lower than the 308.

I have 21.5 mils of elevation left so I could leave it on the 308 zero. Dialing up about 2 mils to zero the 6.5cm and then needing about 9 mils to get to 1000 yards would mean that I would be in the top third of the scopes range with a little left over. So yes, annoying but not the end of the world. This is with a 0 MOA mount.

Still I wonder what others get with their multi-caliber systems. As well is there something a gunsmith do to adjust the POI of a barrel?

Rasyad

 

[Bradu]

In general, when swapping barrels what kind of offset of the zero are you experiencing between calibers, and or barrel length? I am getting a 7.25" offset at 100 yards. Is this typical?

I have a couple new barrels on my gas gun, a 13.5" 308 with a pinned and welded muzzle brake shooting 175 gr FGMM, and a 20" 6.5 CM shooting Hornady 140 gr ELD Match. I have had the rifle to the range a few times and the results are consistent.


Thanks in advance for you help in understanding if I have a problem with my expectations or with my equipment.

Rasyad


View attachment 7104169

What switch barrel system are you using?

 

[Rasyad]

I have a LMT MWS. There are a couple additional twists. First, the service that did the cerakote unfortunately coated the extensions which I then had to very carefully sand off before I could mount the barrels. Second, I filed a small flat (before coating so they show as colored areas) on both extensions in order to clear the M-Loc fittings when swapping barrels. Both of these things may have been boneheaded mistakes on my part and may be factors in the large offset. I simply do not know thus my questions in this post.

Rasyad

 

[newguy2k3v2]

Thanks for that data point, I would have been happy with 1 mil but this is twice that. As well it seems odd that the flatter shooting 6.5 cm impacts lower than the 308.

I have 21.5 mils of elevation left so I could leave it on the 308 zero. Dialing up about 2 mils to zero the 6.5cm and then needing about 9 mils to get to 1000 yards would mean that I would be in the top third of the scopes range with a little left over. So yes, annoying but not the end of the world. This is with a 0 MOA mount.

Still I wonder what others get with their multi-caliber systems. As well is there something a gunsmith do to adjust the POI of a barrel?

Rasyad

At 100yds the caliber and speed difference is moot. It's all barrel harmonics and alignment at that short distance. Based on your calibers and length I'm guessing you're shooting a mws? The 308 barrel could have a slight upward curve and the 6.5 a slight downward and it's just tolerance stacking so to speak.

ETA: it could possibly something to do with you having to sand off the cerakote. It would be interesting if you had another barrel to see where it 0'd compared to the others.

 

[Rasyad]

I have a shop in the basement (lucky me) so I stuck the barrels in the lathe to measure the runout. I did this multiple times in several locations and have reasonable confidence in the results. I understand runout is only one set of dimensions and has little direct bearing on what is going on inside the barrel, nor anything directly about the clamping interface with the upper. Nonetheless, nothing jumped out at me. The 20" 6.5 cm is really straight and the 13.5" 308 pushes down 0.005" at about 12".

So if my 308 barrel deflects 0.005" at 1 foot at 300 feet (100 yards, thank you D1gger!) the POI shift would only be 1.5".

So here are the data points so far:
1. The 20" 6.5 cm barrel is straight.
2. The 13.5" 308 barrel may have a 1.5" downward POI shift of 1.5 " at 100 yards.
3. Cerakote is very thin, typically 0.001"-0.002"
4. LMT uppers are very high tolerance and I had to carefully sand through the thin coating to get the barrels to fit and slip into the upper.
5. The 6.5cm shoots 7.25" lower than the 308.

Of course, I am not a gunsmith and really know nothing of this topic, just a guy fumbling around with his stuff. Still, it is hard to get my head around what might be causing the 7.25" POI shift between these barrels.

Gene at LMT is great and is working with me to try to figure out what is going on. Given that I did modify the barrels this is not about a warranty claim and I am grateful for the problem solving LMT is offering.

I would still like input from the collective experience here on how much POI shift folks see swapping barrels on gas or bolt guns.

Thanks,

Rasyad

 

[theLBC]

how different are the O.D.s of the portions that mate with the upper? is the 6.5 smaller by any measurable amount?
i don't think it can be trajectory, so that leaves the relationship between the barrels and the scope.

 

[Mike_in_FL]

What mount and scope? QD mount? Mount? Just rings?

 

[D1gger]

I'm not a math wiz, but isn't 100 yards 300 feet?

 

[Rasyad]

Gee whiz I am an idiot, edited my post.

Thanks,

Rasyad

 

[Rasyad]

how different are the O.D.s of the portions that mate with the upper? is the 6.5 smaller by any measurable amount?
i don't think it can be trajectory, so that leaves the relationship between the barrels and the scope.

You may be on to something. There is a difference between the barrel extensions. The 13.5" 308 barrel is LMT's ultralite model with an aluminum and steel extension and a pencil barrel profile where as the 6.5 cm is a conventional all steel extension with the standard profile. I will look at this more closely and will confer with Gene at LMT.

Thanks,

Rasyad