Diagnosing a bullet strike

[Tozguy]

With todays choice of bullets and powders for a given calibre it can get tedious and expensive to test all the possibilities. For two of my projects, I consulted the bullet techs at Sierra and they helped me to narrow the choices down, to adjust my expectations and to see when a certain calibre just does not have the potential I was looking for.
But I agree with Dabbler, experimenting is loads of fun when it gets fun results.

 

[wileel]

cdhknives, the only way we ream chambers and crown muzzles and thread barrels is using a steady rest... My toolmaker friend and I tried a bunch of other approaches, but this way we can achieve tenths concentricity and perfect alignment to the bore - both of which prevent problems such as you are facing. Because we are so careful we install all muzzle breaks and suppressors for out city's tac team - they just don't trust anyone else.

BTW you don't need contact to flip your bullet... we tried a few off centre muzzle breaks with .020 clearance to the bullet... With .005 out of concentricity the bullet yaws considerably at 100m. I presume it is the differential gas pressure/gas velocity that does it.

Beat me to it! This is something that can easily be overlooked...esp when dealing with a can that is all about gas pressure

 

[cdhknives]

As for experimentation, here is a less than exhaustive list:
110 RN (Cheap, 30 carbine bullets)
110 Barnes
150 RN (Herters)
170 RN (Herters)
220 RN (Sierra)
150 Spitzer FB (Sierra)
178 AMax
180 Nosler BT
200 Sierra Pro Hunter
220 Nosler match
220 Berry's plated
220 Sierra Match
240 Sierra match
245 Missouri Hard Cast coated

Powders, H110, CFE BLK, 1680, Trail Boss

I didn't get serious about subs until a few months ago. The light bullets were full power supers. Full power loads all seem ok. Average at best accuracy, but no tumbling/yawing.

I bought a cheap USB 5.5mm bore scope. Not great out of the box. With a little work and a good cleaning of the lens (plus additional light into the bore) I can get a workable picture of the bore.

No obvious defects. Smooth and shiny. The gas port is drilled right through one rifling land. 5 groove rifling. It seems fairly uniform from end to end.

A very tight patch is noticeably looser (but still not 'loose') in mid bore. There is nothing in the camera view to support this. That is literally the only defect I can find in the whole thing that could affect bullet flight.

Maybe I should try catching a bullet in water jugs and see if there is evidence of skidding on the rifling. I tried to dig one from the berm and they were 12"+ deep in hard clay mud.

I have also been shopping for new barrels. I still want to learn why this one is so bad...for my education if nothing else...

 

[Dabbler]

You have certainly tried... we were lucky to get good results without that much work!

-- By pushing a soft brass plug through the bore you can ascertain the variance between the muzzle diameter and the constriction. (Do not attempt this if you don't know the technique!) If it is more than a tenth, you have a bad barrel. Even if you've had it for years, the better barrel manufacturers will want to do something to help you out - purely for professional pride.

One notable exception is the new breed of carbon fibre barrel builders. For about a year we got in several very bad carbon barrels, and the manufacturers (there were two) gave answers that ammounted to "so what?" We no longer use barrels from those suppliers.

 

[pdentrem]

You could use a softer lead ball to do the same as the brass one.

 

[cdhknives]

I am familiar with slugging a barrel with a lead slug. I am unsure how this will show a variance in bore. It typically is understood to give the smallest diameter inside the bore as the plug is swaged down.

 

[pdentrem]

As the slug is pushed down the barrel, it will be hard on the start but after that it should be constant in the force required. At an area where the barrel is tighter or looser the force will go up or down. The slug will measure the smallest naturally, and you can then reintroduce the slug and it will slide easiest in the areas that are larger and you will be able to have a better feel of the interior and the place where it is loose. If it is a button rifled barrel, there is not much variance from one end to the other. At the most a mere 0.0001” or 2 open taper in the direction of travel that the button took through the steel.

 

[seasicksteve]

Im thinking the 8 twist is not enough twist to stabilize those long 220 gr bullets at subsonic velocity. Perhaps a 7 twist is in order
GM 7 twist 300 BO barrel I put that 220 gr nosler into the greenhill formula at 1120 fps I got 1in7.3 twist rate

 

[Dabbler]

cdhknives, after you pass a slug down the barrel, you can use a blade micrometer to check what the smallest bore dimension is. Compare that to the muzzle bore and you might have found the problem. Just another check. I've had more luck with hard lead or soft brass slugs, but some guys have found soft lead to be fine.

 

[cdhknives]

Steve, the question I have is whether the Miller and/or the Greenhill is accurate for subsonic velocities. I can find nothing in basic internet searches that addresses this question. The Miller formula per the site linked a few back says the Nosler 220 Match should be fully stable down to 500 fps. There are an awful lot of 1:8 barrels for 300bo on the market that guarantee use with subs...I just don't know. They are both related and both are empirical in nature, but if I read my history right both were generated for high velocity use. I have enough education and experience to understand fluid flow at subsonic velocities and supersonic (laminar vs. turbulent, boundary layer flow, etc.) but introduce spin stabilization and all bets are off. I get the concept, the application is not something I can evaluate better than Miller and Greenhill for sure!

Dabbler, I have found hard lead to be a bit scary to get started in a bore. I don't like hammering it in to start...always afraid I'll ding the bore with the driving rod even if it is brass. That said I have some 30:1 casting alloy I could try in addition to ample dead soft pure lead. It might not happen till after the holidays though...as much as this is stimulating my engineering curiosity, family priorities are about to take over!

Merry Christmas to all y'all from South Texas!