First time I've seen this but have heard about it. The guys at the range had never seen it before either.
Keyholing and the same bullet in another two-groove.
Note the single bullet that did spin up.
It obviously to me that the bullets were skidding the rifling and simply not spinning up to speed. It just so happens that I know about skidding of the rifling in a two-groove, having done some tests in the past.
So I took it home and considered relegating it to the scrap pile (or changing the barrel). But, I did some bore diameter measuring and found that the muzzle was trumpeted - which I knew about, it having had some bore rust, mostly at the muzzle end. I also found that the bore was tight in the middle area. It was also a bit rough in that area.
I first fired an identical load and found that it skidded and went through the catch medium sideways.
Then I lapped the bore until I could no longer feel any difference in resistance throughout the bore. I then counter-bored the muzzle up to the point that I had found the trumpeting to begin.
Then I did some tests. Now at this point, I had been reading up and the consensus was that some 303's just don't like boat tail bullets and that they will sometimes keyhole with them. The wisdom is to use flat base bullets. I can understand the flat base bullets being more accurate than boat tails b ut keyholing? Well yes, if one sees what's actually going on in the bore. But why this two-groove and not my other one?
Well, here's why. The guilty two-groove has a larger bore and a restriction in the middle area. This translates into rifling skid. Now, sometimes a skidded bullet will eventually spin up and exit the muzzle following the rifling. I know this because I have done tests and recovered the bullets.
One can see by the striations that these bullets did eventually spin up.
Apparently, what happens is that a while a boat tail might skid the rifling, a flat base has the more substantial base which will engage the rifling. Then again, a skidding bullet is still undergoing a spinning force as it progresses down the bore. In this particular rifle however, with a narrower bore which then opens up a little, only by .001", loses its grip on the bullet so it never spins up fully. Now, I can tell by the target impact that there is some spin in the bullet because they are hitting sideways and not just tumbling head over heel.
So after lapping the bore, I fired a few bullets down the bore that I knew would skid. And as I expected, they had spun up fully and penetrated the catch medium nose first. I then used a powder that produced a lower pressure and an undersize bullet (308 in a 303) and it engaged the rifling. Next, I increased the charge to get it to skid which it did but it still spun up. I then loaded a .311 bullet and fired it and it engaged the rifling. This is with flat based bullets. I am yet to put a boat tail through it after the lapping.
This is what a fired bullet should look like.
This by the way, is a .308 bullet in an oversize 303 bore. The second skidded bullet above was loaded to the starting load.
The takeaway here is that a keholing problem can be fixed by ensuring that the bore does not have a counter taper or constriction in the center area and by reducing the powder charge to the point where skidding does not occur. This applies to cast boolits. I had previously fired cast boolits in this rifle and they went through just file but with low powder charges. I would suspect that with the trumpet muzzle, accuracy would have been poor.
Here one can clearly see full engagement of the rifling with no skid at all.