I have an Ed Rayl barrel I’m trying to understand better. It is a .47 caliber 7-groove (round bottomed grooves) barrel and I can’t figure a method to accurately measure the depth of the grooves. Internet searches come up with “slug the barrel, wrap shim material around the slug and do the “πR²” math to get groove to groove diameter”, which makes perfect sense, assuming one has the ability to slug a muzzleloader, perfectly wrap the shim and accurately measure the resulting circumference…not me, thank you very much. Is there a better way?
Several measurements taken with a caliper from the (best effort) center of the land to the bottom of the opposite groove (remember this is a 7-groove barrel…it is not possible to get an accurate land-to-land or groove-to-groove measure with calipers directly) results in .470 often enough to lead me to believe that is an accurate measure. I would expect that measure (since the land-to-land bore measure should be .470 in a “true” .47 cal barrel) to result in something more like .482, assuming a .012 groove depth.
Am I missing something here? See below for the load I've settled on, which would add up to .496 (cast balls pretty consistently measure .460 + ticking (.018 x2 = .036) = .496) compared to assumed groove depth of .470 + groove depth (.012 x2 = .024) = .494. This provides a very modest.002 compression. The loading feels a little tighter than that, so it wouldn't be a great surprise if the bore actually does measure less than a true .470, or perhaps the grooves are more shallow than my assumed .012.
Also, I may have totally fouled up the way I'm going about these calculations.
This is all academic, although I’m curious how much compression I’m getting on my patch material. I can’t shoot one hole groups at any distance (I’m getting senior), I’ve already arrived at a patch/powder/ball combo I like and I’m not one to spend days and days trying out infinite combinations anyway. But I’m curious.
If you’re interested, here’s what I settled on: TOW ticking, reportedly .018, pre-lubed with melted mink oil and squeezed out, because I’m a hunter, not a target shooter; 60 gr FFFG; .460 ball, mainly because I found a used .460 round ball mould that casts pretty balls that release easily from the mould. This combo loads easily, recoil is pleasant for a rifle this light (7lbs on the nose), fired patches show no signs of stress, it’s more accurate than I am and should pack enough energy for Tennessee whitetails in the woods. Last but not least I don’t want to push too hard on a really, really skinny swamped pipe (.8 across the flats at the muzzle). All the shots feel about the same going down after the first one, without wiping and clean up about the same using greased patches vs moose milk.
I tried a thicker denim patch (won’t try to supply a measure because I couldn’t get the same result twice trying to squeeze calipers around a ball patched with the stuff…it was pretty thick). It grouped about the same as the ticking (diminishing returns with 60+ year old eyeballs), it was harder to load (it would have stressed that skinny stick if I’d tried it…I used a stainless range rod instead) and my wife found it at JoAnn’s. Who knows if I could have laid my mitts on any more of it. I also tried loads up to 75 gr of FFFG, but didn’t like how the rifle started to buck. I'll tolerate some kick if there's a good reason for it, but I don't invite it for fun.
Thanks for any comments or ideas about groove depth. It’s a new rifle (an Ian Pratt rendition of a J/E Bull brothers E. Tennessee rifle) and I’d like to document all the niceties before I move on to something else.
The requisite picture. The target (at 25 yds) shows two three-shot groups, a 60 gr load more or less at point of aim at 6 o’clock, and a 50 gr load just below that, and a 30gr squib load just off paper at the very bottom of the pic because Daryl said that would make cleaning a little easier (not so’s I could tell, though). I’ve got to bump that front sight one more time.
