looking at have a 6mmai build for my grandson 16 birthday. looking for some ideas on barrel length and twist for any 85gr tsx bullet. The gun will be mostly used for deer.

The 6mm A-I and the 85 gr. Barnes TSX should perform well with a 1-10" twist rate....

A

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The standard for the parent case is 1:9, which will stabilize all but the longest target bullets in that caliber. The slightly slower 1:10 twist should allow a bit more velocity to be realized, but you're really splitting hairs on that count. One of those two will certainly be suitable.

I used a 1/10 twist barrel in my 6RemAI @ 25" long barrel. I also use 1/10 twist in my 243AI and 243.

I've never shot the Barnes bullets in my 6RemAI mainly use that rifle for antelope with 95gr Partition.

Nosler making a 90gr E-Tp and Berger is coming out with a 87gr VLD hunting bullet for the 1/10 twist 6mm barrels.

I've done in 6mm 1/8,1/10,1/12,1/14 twiast barrels. If your building the 6RemAI on a short action I'd do the 243AI get alittle more magazine length. Sounds like a good project

I inherited a 6mm AL Mauser, Timney Trigger, 24"Broughton 1:10 twist

47gr h4831 sc 3005 fs 95gr NBT shot lots of deer and coyotes.

47.5gr h4831 sc 3120 fs 85gr tsx barnes very accurate
Shot a doe at 75 yds headon bullet weighed 83gr bullet traved

Really fun to shoot.

I too would use a little faster twist like 1:9 or 1:9.25. At some point when your grandson gets older he might want to use a heavier and most likely longer bullet in this gun. I have a Savage 243 and they twist their 243's with a 1:9.25 rate. I strongly believe this helps with the heavier bullets that are longer with the high BC's. I'm sure he'll love it !!

I'm with old roper, except I'd do it in a .260 Rem instead.