Matching Alloy Strength to Pressure

[303Guy]

65714

Here we have two boolits of the same shape and weight fired with the same load into the same catch medium. The difference between them is the alloy strength and hardness. If one looks closely one can see the top one has what looks like a narrower middle. Actually it is not narrower than the front but simply has no knurling. The boolit has a two diameter form with a taper step. Note the rifling impressions in both boolits. The bottom boolit has rifling impression almost up to the nose while the top boolit has rifling impressions to the middle of the step down taper.

Here's what's going on. The top boolit has not undergone any bump up while the bottom boolit has bumped up to just before the nose. That represents the optimum alloy strength for the pressure generated by the charge. The first boolit can be expected to have poor bore alignment and hence poor accuracy. The bottom boolit on the other hand has had just sufficient bump up to support it in the bore and align it. This is what I mean by balancing the alloy strength to chamber pressure. But what about too soft an alloy?

The nose gets bumped up as in 'nose slump'. So the optimum alloy strength or hardness will allow the bullet to obturate or bump up fully without nose slump. (Forgive the oversize photo - I'm struggling with the new look, impossible to work with photobucket!)

http://i388.photobucket.com/albums/oo327/303Guy/Riflingskidnoseslump002.jpg

These were paper patched boolits which is why the noses were under bore diameter.

[waksupi]

My concern on examining the boolits, is the uneven base on the lower example. I would think this would lead to a less accurate projectile?

[303Guy]

Yes, boolit base distortion is a real concern. However, this one was due to impact damage. I'd quickly take and post a photo of the base but I'm using a camera with very poor close-up ability and it takes about 10 snaps to get one that's almost usable. (The second photo was from my old camera - I miss that camera!)

[DrCaveman]

What are the alloys in the example? And approx pressure of that load?

[303Guy]

I wish I could answer both questions. I don't even know the hardness. I do know the relative hardness by feel and that's all. These are copper tin alloys I've been playing around with. On pressure, the only thing I can say is the powder was 6gr Trail Boss with a tuft of Dacron and the boolits were 206gr. The second and softer of today's boolits seemed to produce slightly higher pressure and velocity. Put it this way, the primer indent shows hardly any pressure at all. The bottom photo is an older one with possibly a similar alloy to today's softer one but the powder charge would have been higher, possibly 10gr TB.

[44man]

3, you sure shoot some funny boolits!:kidding:
Which direction do they spin since you are in the upside down part of the earth? :bigsmyl2::bigsmyl2:

[Doc Highwall]

Now that you mention it 44man it is a left hand twist.

[303Guy]

OK, here're some pictures of that boolit base.

6577465775

As can be seen, the boolit base is just about perfect with no pressure induce base distortion, no cupping, no metal dragging, no feathering - just perfect. But will they shoot straight - that is the question! At least now I have a starting point.8-)


P.S. How hard is it to attach a strain gauge to the barrel and connect it up to a PC so I can take comparative pressure traces?

[Doc Highwall]

303guy, you need a little more then a P.C., I have attached the web site where I purchased mine.
http://shootingsoftware.com/

[303Guy]

Thanks. That would be a useful tool. No spare cash at the moment, unfortunately. That secondary pressure spike is scary!

[303Guy]

I've done another test with a harder alloy with a higher pressure loading and plain cast. The base deformed too much to be worth range testing but what struck me was that there is visible signs of partial rifling skid yet the boolit obturated to reseal the bore. The test was more about plain cast in that particular bore than alloy strength. There is leading so perhaps there is lube failure at play here too.

6581965820

[runfiverun]

unhunh.
gas cutting.

i'm more fascinated by the ring around the boolit in the second pic in the first post.

[geargnasher]

Tin/copper alloy? Any lead in there? That bore fouling might be "tinning" instead of leading, which can happen purely from abrasion regardless of lube or obturation. Rough bore will make it way worse. It even happens in overtinned lead alloys containing some antimony.

That ring looks like either crimp impression or a patch wrinkle from patch slippage upon engraving.

Gear

[303Guy]

There is quite a bit of tin in the first two. That ring is a casting flaw - I use flawed castings for 'test tube' tests since accuracy is not an issue. I was getting rings like that when the lead stream ran down centre into the mold.

There's no sign of gas cutting on the last plain boolit and indeed there shouldn't be since I used wheat germ filler. I don't recall ever seeing gas cutting with wheat germ. Even when there is some slip there is no gas cutting. I'd say gear is right about the tinning. That would explain the full obturation.

Having a closer look at that last boolit pic I see what looks like gas cutting.

65862

It's actually a flaw in the bore on one of the lands that leaves that impression. It's visible all the way down the bore. That particular land is wider than the others. I have a faint suspicion that the chamber is misaligned too. I'm prepared to accept that this rifle will never be a shooter but I'm sure going to try just the same.

[runfiverun]

if it's a consistent flaw all the way down the barrel you should be able to work around it.