Case Wall Variance and Crimp

[martin]

Hello,
I was bored and started measuring the case wall thickness at the mouth of some new Starline cases in 40 S&W. I found that the thickness varies between .011 inches to .013 inches. I set my crimp for .421 inches - max tolerance is .423 inches for 40 S&W. My sizing die reduces the diameter at the mouth to .417 inches. I do not bell the case mouth. I am using a Berry's 135 grain RS style bullet which has a bullet diameter of .401 inches - no real variance there. I am using a Dillon 550B with Redding Carbide dies.

I measured some LOADED Winchester ammo for a sanity check and at the case mouth it measures .418 inches. I have tried to simulate setback with my rounds by pressing the bullet against the bench and have not been able to get any setback at all.

With the case wall variances I am seeing the effective crimp will be greater or less depending on the wall thickness. Based on my crimp diameter of .421 would you consider my approach -
1. Acceptable
2. Lame
3. Sell the gear and move to Cuba?

Regards

 

[stans]

I gotta go with option number 3. Seriously, a thicker case mouth is going to result in more crimp. Does this really mean anything? It might if you are using mixed brass, but I think if you segregate your brass according to manufacturer you will be just fine. I have loaded thousands of rounds of 45 ACP and 9mm. My ammo is usually very consistant except when using mixed cases.

 

[martin]

I don't worry about it with 45 or 9 either. These little 135 grainers are going to be pretty hot out of the 40. I'll see tomorrow what I get. I loaded up a variety using N340 and N320. The range will be packed tomorrow so I won't be able to break out the chronograph. I'm anxious to see what the 135 grainers will do.

 

[WESHOOT2]

OH THE HUMANITY!

In auto cartridges suggest using crimp to remove mouth flare; use case neck tension to secure the bullet.
Testing required; accuracy, you know............first given that your load feeds, fires, and ejects with perfect (as in 100%) reliability.

 

[martin]

The N340 was a definite maybe yesterday. Of course, the N320 was "marginal" to be polite. Off to try some Power Pistol and Universal loads with the 135s.

The N340 best grouping from bags was .5 inches ctc with 5 shot groups. I will need to load up more to test for velocity. 5 shots isn't sufficient to make a decision by any means. 10 shot groups will give me a better idea.

BTW I pulled one of the Winchester loads I mentioned in my first post. There was some serious bullet deformation from their heavy crimp.

 

[Sunray]

Use your barrel's chamber as a guage. If the loaded ammo drops into it, it's fine. You don't need to measure it at all. Taper crimp for autoloaders.

 

[martin]

My question was really aimed at getting some feedback on the impact of case wall variance and its effect on the pressures seen in the 40 S&W cartridge.

A usual press fit would be considered pressing an oversized object into a smaller hole - the oversized object being .005 inches larger than the hole it is going into.

So, if the case wall thickness is .011 inches, the hole inside diameter will be .395 inches after resizing(first post indicated resizing leaves case with an outside diameter of .417 inches). The equation is:

outside case diameter - (2 x wall thickness) = cartridge hole diameter

The case with a wall thickness of .011 inches will be considered slightly greater than a press fit since the bullet diameter is .006 inches larger than the inside diameter of the thinnest case. After bullet seating with this case, the outside diameter of the case mouth will be .423 inches. This is the maximum SAAMI specification for outside diameter and for safety purposes should probably be reduced by .001 inches to .422 inches.

The case with a wall thickness of .013 inches will leave a hole diameter after resizing of .391 inches. This will be substantially greater than a press fit since the bullet diameter is .010 inches larger than the inside diameter of the thickest case. After seating with this case, the outside diameter will also be .423 inches - with substantially greater neck tension.

Looks like I answered my own question in a roundabout way. I really need to insure that I bell the case mouths so that all cases have the same neck tension regardless of case wall thickness - that is the important thing to consider since we will always be reducing the case diameter to .422 inches to insure we conform to the SAAMI specification. The important measurement is the inside diameter of the case to insure there is .005 inches difference between the bullet diameter and the inside diameter of the case. So, with a bullet diameter of .401 inches I need to bell my cases so that the inside diameter of all cases is .005 inches less than the bullet diameter = .396 inches.