Bullet Obturation and Rifling Groove Depth

Sorry Kyle... I had land and groove bore diameter on the brain.

I meant to say standard groove rifling with 0.004" and reduce that to something on the order of 0.002".

I was really eluding to better bullet obturation into a shallower .002" groove as scat has stated (better gas sealing, possible increase in velocity).

Obviously, a full-form sized bullet would pretty much eliminate the need for this BUT the bullet will have additional frictional drag on the jacket in a deeper groove.

On the subject of cut rifling depth, variation is groove depth is not a new tooling proposition.  Cutters can be shortened (height) by a variety of means to create shallow cuts.  More of a setup operation. Button rifling.. well yes, the button head would need to possess shorter engraving protrusions so there would indeed be tooling modification there.

Dec 7, 2017 9:11:25 GMT -5 hillbill said:

the Rock Creek barrels shoot very well indeed, we already know that but their grooves are only half a thou shallower than normal.

I really don't think the slightly shallower grooves attribute to their top notch barrels But I do think the narrow land 8 groove profile does help.

every barrel of theirs I have used so far (3) have spun very true after being cut to length, you can't say that about a lot of barrels, even the high end barrels. That tells me the ones I have are drilled very straight.

Thanks for the reply hillbill. Obviously, RC barrels are top notch.

On the subject of gundrilling, I can explain how a local Indiana gundrilling company actually performs the operation and what to expect in terms of barrel straightness.

During a competition shotgun barrel building project I completed in 2013, I used a dedicated gundrilling company named Davis Machine & Tool in Edinburg, IN to drill out four 41L40 1 1/2" round bars (36" in length) for me (.726").  The owner was very nice and allowed me to watch the actual drilling of one of my barrel blanks (very cool to watch I might add).  After one of the barrel blanks was drilled, I inquired as to what kept the drill bit directly in the center of the blank for the entire length of the blank?  I was surprised at the answer:  The drill bit DOES NOT stay in the center of the blank as it drills its way to the other end.

It was explained to me that this is quite normal for gundrilling operations.  More specifically, the driller told me that after the entire 3"-4" length of the drilling head is into the metal of the blank, the drill bit can "walk" in whatever direction it chooses.  Interestingly, the bit "walks" in one direction the entire length of the blank.  Further, it was explained that it is typical for the bit can walk (i.e. drift) off of the blank's centerline by .001" for every linear inch of barrel blank!

So for my 36" long .726" bore barrel blanks, the hole on the other end of the barrel could be up to .032" off of the centerline of the blank!!  This sounds like a problem but , in reality, it isn't (provided that you oversize your barrel blank to start with).  My four 36" barrel blanks varied in amount of gundrill bit drift from .014" to .022" off of the blank's actual centerline. 

So why no big deal?  Again, oversized blanks for this project "save the day" but DO require a post-gundrilling operation:  turning the barrel blank on centers in a lathe.  In essence, cut the exterior of the barrel to match centerline of the drilled (and honed) hole in the blank.  Obviously, turning on centers is best accomplished AFTER honing the bore to uniformity. (there ARE OTHER TECHNIQUES that can be used to the same effect but what is in this post is what I observed firsthand).

I understand that each barrel manufacturing shop has there own methods, ways, and secrets for claiming their barrels are superior to the next guy's... that's normal marketing (and may be true in some cases).  What is never really published on websites in literature is the finer details of how the manufacturer ensures that the barrel bore is concentric with the bore.  Some accomplish this via turning the outside of the barrel concentric to the centerline of the bore after gundrilling and honing while others use "straightening machines" that effectively bend the blank straight within a tolerance range.  Probably other techniques too but I tend to gravitate towards machining (less stress on the metal itself).