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Does a shotshell roll crimp raise or lower pressures versus a fold crimp?

Posted on December 14, 2012   by Ballistic Products    
 

Roll Crimps vs. Fold Crimps: Pressure differences

We are often asked, “Does a roll crimp on a load raise or lower pressures?”

The answer from our Lab:
 It is impossible to test every possible case and combination of fired hulls, etc. Shotshell tests are known as “destructive tests.” This means that they can never be duplicated again and generalizations cannot be derived from perceived patterns within destructive tests.

We do know this: When roll crimping a new hull (i.e., a hull that has never before roll crimped or fold crimped) and given an equally-loaded and identical load that has been fold-crimped, the roll-crimped load never seems to achieve the pressure level of a fold crimped load. This is to say that lab patterns show that roll crimped loads in parallel load comparisons generally demonstrate lower pressures. Remember, this pattern is only with the loads that the ballistic lab has fired. The ballistic lab has NOT tested every possible load combination with this methodology.

Here is a list of tested examples that our lab has compared:
◾FOLD-CRIMPED LOAD:
16GA 2.75”    Longshot       XX.X  Z16               7/8oz  ITX6    10500 PSI     1540 FPS
◾ROLL-CRIMPED LOAD:
16GA 2.75”    Longshot       XX.X  Z16               7/8 oz ITX6     9000  PSI     1445 FPS
◾FOLD CRIMPED LOAD:
12GA 2.75”    Red Dot         XX.X   PISK-LITE     3/4oz Lead    10000 PSI      1600 FPS
◾ROLL-CRIMPED LOAD:
12GA 2.75”    Red Dot         XX.X  PISK-LITE      3/4oz Lead     9500 PSI       1560 FPS
◾FOLD CRIMPED LOAD:
12GA 2.75”    Green Dot      XX.X   PT1205         1oz Lead        8100 PSI       1225 FPS
◾ROLL-CRIMPED LOAD:
12GA 2.75”   Green Dot      XX.X  PT1205           1oz Lead        8000 PSI       1210 FPS
◾FOLD CRIMPED LOAD:
12GA 2.75”    Clays              XX.X  PT1215         1oz Lead       10900 PSI      1380 FPS
◾ROLL-CRIMPED LOAD:
12GA 2.75”   Clays              XX.X   PT1215         1oz Lead        9200 PSI       1315 FPS

Do we know their testing protocol? There are just tons of assumptions that I must make to trust this data.

How many shots were taken to establish each pressure benchmark?
How closely are the reloading protocols followed for each and every shell?
Are the cases all from the same lot? Is the plastic for these case runs in the same lot of plastic production?
Did they use a control shotshell for each new day of testing to determine environmental changes?
Is this just the mean for each load? What is the maximum variance for each load?
Have they used different equipment for roll crimping and fold crimping? Ideally they would use a selection of fold crimping tools and roll crimping tools.

These are just a few things I can question in just seconds of thought given to this test. It is exceedingly rare for any 'gun' test to be anywhere close to a useful data collection test much less a truly scientific test protocol/data analysis procedure. 

Plebian: You are right that many protocols and even the powder is an unknown factor.  However, this is presented as a general statement that "... given an equally-loaded and identical load that has been fold-crimped, the roll-crimped load never seems to achieve the pressure level of a fold crimped load."

cpaspr: You are also on point, the conclusion does seem to be what we would expect.

Now would it also be expected if the premise instead were to read like this?
"... given equally-loaded and identical fold crimp loads that have been assembled in  2.75" and 3" hulls, the 3" loads never seem to achieve the pressure level of the 2.75" loads."

Here is an *example example of the alternate premise:
First and third lead shot loads on the page - both have the same data, both are fold crimped, but one is a 2.75" load and the other is a 3" load.  The 3" load exhibits lower pressure and velocity.
https://www.ballisticproducts.com/VP53%20data.pdf

*Note: This copyrighted data would not properly format, so an off-forum reference is shown.

To cut to the chase: What was not spelled out in the 2.75" Roll Crimp vs Fold Crimp example is simply the Loaded Length of the Roll Crimped load is longer than a Fold Crimped load.  The Roll Crimped loads have a greater loaded internal capacity. So the comparison is not an apples to apples example any more than comparing the same data put up in 2.75" and 3" fold crimp cases.

For the differences between a roll crimp and fold crimp to be properly compared, the roll crimped hulls would have to be shortened.  That way the finished loads would have the same internal loaded volume.

A loaded 2.5" roll crimped cartridge would have the same loaded internal capacity as a loaded 2.75" fold crimped cartridge.   

One of my shotshell handloads was designed to use a steel shot type wad and 16 thirty* caliber, 40 grain, high animony lead pellets; stacked in layers of two. This is just shy of 1.5 ounces.  This load is a perfect fit in 2.75"  Reifenhauser type straight wall hull using a roll crimp. Using a fold crimp the load fits perfectly in a 3" version same type of hull.   

So why use the "old fashioned" roll crimp at all.  Well the old 2.75" chambered Remington 1100 has the action length to feed the long roll crimped length of this tight patterning load. The roll crimp allows me to fit the load to the gun.

*#1B is nominally .30" but commercial loads are assembled with pellets closer to .286" and 33 grains each.

The average case length  is .40" for folded crimp vs .15" for rolled crimp. 

So just a bit of illustration:

2.50" - .15" = 2.35"    Roll crimp 2.50" hull loaded length is 2.35"

2.75" - .40" = 2.35"    Fold crimp 2.75" hull loaded length is 2.35"

2.75" - .15" = 2.60"    Roll crimp 2.75" hull loaded length is 2.60"

3.00" - .40" = 2.60"    Fold crimp 3.00" hull loaded length is 2.60"