I think that generally a groove diameter of .380" is the most common among .38-55 rifles. But as you mentioned they vary a fair amount from that. But tools and techniques used in rifling a barrel will still change today, so you might order a barrel specified as .380" and get a .379" or a .381" It's just not that precise still.
Twist rates used to commonly be around 1:20" for makers in the late 1800's and early 1900's. And that was for the typical 255 gr. bullet. Custom barrel makers like Schoyen and Pope usually went with a 1:15" twist for heavier 300-320 gr. bullet use. I think many barrel makers today are also using this faster twist rate. I see Green Mountain, and Pedersoli both make barrels with a much faster 1:12" twist today!

And a 15 twist is about right based on my own personal experiences. Most barrel makers will typically note at least their actual groove diameters or you can just simply call and ask for both.   

I built a RB in 38-55, Douglas .375 1/16 twist. When/if the next time comes around I will not use a 16.

marlinguy wrote on Apr 5^th, 2021 at 10:59am:

0.380" may have been a standard 100 years ago, but 0.366"x 0.375" has been the standard from GM, Shilen, Bartlein, Krieger, etc., for as long as I've been shooting 38cal. 

And yes, 12" twist is the most successful for longer range shooting (BPCR & BPTR), though several folks are starting to shoot 10" twist barrels with their 38-50 & 38-70 rifles with the longer/heavier bullets. I'll be shooting a 343gr DEPP bullet in front of 50gr of 2F Swiss this weekend at our 200/300/600yd mid-range match. Not sure if that is enough umph for the bullet in a 12" twist barrel. May have to rechamber to 38-50 where I can easily get 65gr of powder behind it.

Was shooting 54gr of 2F Swiss in my 38-50 with the 360gr bullet.

rgchristensen wrote on Apr 9^th, 2021 at 2:48pm:


Have a similar issue with my Hepburn Match B schuetzen rifle in .32-40 chamber. The groove is .327" and it wont chamber fixed ammo. So I had Jerry Cleave build a breech seater for it, and just shoot it as a breech seater.

Any cut rifled barrel maker worth his salt should be able to make whatever you desire, or are willing to pay for. Time is Money, and for the barrel maker to deep hole drill to a specific size, and the necessary cutter/s for a specific groove size takes time, as does hand lapping etc. The old saying, You get what you pay for, applies here. My guess is that some are so busy and behind on orders, they just can't or won't take the time for specialty work. Most probably decide on a standard(for them) bore and groove diameter and don't have the time or inclination to vary from that.