Fold Crimp versus Roll Crimp

[Graybeard96]

If I understand my Books correctly, more pressure is created using a Fold crimp then a Roll crimp. Why would that be ? Seems to me exactly the opposite is true.

All a Fold crimp has to do is open up the Petals, whereas a Roll crimp needs to "unroll" which to me seems a lot more effort, thus creating more pressure.

Any thoughts on that ?

Cheers

[Shawlerbrook]

Your logic makes sense to me. I will be awaiting the explanation.

[centershot]

Sometimes science trumps "sense"; Pressure testing proves that fold-crimps produce more pressure than roll crimps.

[Shawlerbrook]

Care to explain the “science” ?

[megasupermagnum]

The only thing that I can come up with is that the rolled lip on a fold crimp is far tighter than the more gentle turn over of a roll crimp. The more peculiar thing is that the depth of a fold crimp can make a significant difference on pressure. The depth of a roll crimp does not seem to matter much at all. For the most part, the pressure difference between a reasonable .055" to .075" deep fold crimp, and most any roll crimp is pretty minor in any testing I've done. I've not seen more than a 500 psi change, but that's not to say it can't. I don't think I can recall a roll crimp ever testing higher pressure than a fold crimp. Maybe if you had a really loose junk fold crimp.

[Graybeard96]

The only thing that I can come up with is that the rolled lip on a fold crimp is far tighter than the more gentle turn over of a roll crimp. The more peculiar thing is that the depth of a fold crimp can make a significant difference on pressure. The depth of a roll crimp does not seem to matter much at all. For the most part, the pressure difference between a reasonable .055" to .075" deep fold crimp, and most any roll crimp is pretty minor in any testing I've done. I've not seen more than a 500 psi change, but that's not to say it can't. I don't think I can recall a roll crimp ever testing higher pressure than a fold crimp. Maybe if you had a really loose junk fold crimp.

I wonder if you could share Charts showing actual resistance testing for both Fold & Roll crimps ?

Thanks

[Polymath]

I have thought that too. however I use those BP frangible discs in 12 ga. In the other that require roll crimps I use my home made cards. Some plastic, some waxed paper.

[Squid Boy]

I recall an experiment conducted by one of the contributors to the British sporting paper called "The Field". I have a book of these articles by a Frederic Toms and as I remember they took various shells and made a plug to fit inside the hull with a thin wire attached that went through the crimp. I believe they put it in a barrel and pulled the wire with a scale and took a reading when the crimp gave way. Roll crimps were weaker than folded but a lot depended on case condition, depth of the turn-over, etc. My experience has been that deep fold crimps have an effect on increasing pressure. I will often deepen crimps on shells that show more variation and it will sort that out most times. So like most things, everything depends. I will post it if I can find that article. Thanks, Squid Boy

[megasupermagnum]

I wonder if you could share Charts showing actual resistance testing for both Fold & Roll crimps ?

Thanks

I doubt any charts exist. All I can show you is pressure testing. This was my most recent. https://castboolits.gunloads.com/sho...52#post5472352

[toot]

I always use a roll crimp on my 12 & 10 gauge BP, hulls. and I also am of the thought that a roll crimp creates more resistance unrolling when fired. but the old BP hulls of years ago were done in that way, rolled crimped.so?

[toot]

what is used in CASS & SASS, shotgun shooting coemption? and do they use black powder, or both? just courious?

[centershot]

Care to explain the “science” ?

Pressure testing.

[Shawlerbrook]

I know how they measured it, but can someone explain why folded causes higher pressures than rolled ? If you go back to the OP you will see that was the question. Sort of like I know you use a thermometer to measure it’s cold, but please explain why...it’s winter and the sun angle.

[trapper9260]

If I understand my Books correctly, more pressure is created using a Fold crimp then a Roll crimp. Why would that be ? Seems to me exactly the opposite is true.

All a Fold crimp has to do is open up the Petals, whereas a Roll crimp needs to "unroll" which to me seems a lot more effort, thus creating more pressure.

Any thoughts on that ?

Cheers

From all I had read that roll crimp is more pressure, also when factories start to load shells fold crimp was faster the roll. I know it takes more to undo a roll crimp then fold and also you see factory slug loads use roll crimps because from what I know you need more pressure . Unless someone else know something different .

[Shawlerbrook]

Could it be that the tests were on factory loaded ammo where the folded crimps are sealed by that circle of plastic in the center of the crimp ?

[pmer]

I wonder if the over shot card bursts out quicker as ignition of the charge gets underway?

[Cap'n Morgan]

A roll crimped shell have a longer OAL than a folded crimp. With everything else being equal, this means a more compressed wad = deeper setting = increased pressure.
I strongly doubt the actual force needed to open a crimp has any direct bearing on the pressure, though it may indirectly do so by improving the ignition - like a hard crimp on a pistol load.

[Graybeard96]

Thank you "Squid Boy" for your comment and providing an Idea how to make a simple Instrument measuring resistance of Fold Crimp versus Roll Crimp.
First Step is putting a Strong String with Loop trough the Primer Hole so the Shell can be hung upside Down.

The last 1/2 inch ID of a brand new W-W 12 Ga. AA Hull measures .750 thousand. A 2007 Canadian 1 Cent Coin made out of questionable "Steel" measures .750 I will drill a small Hole in the Center allowing a tough Wire to pass trough to which I plan to attach a empty Jug to be filled slowly with Water. The Water amount weight difference to make Fold or Roll Crimp to open up can then be easily calculated and if there is any difference will show which Crimp takes more effort to open.

Perhaps next weekend I have time to work on this little experiment.

Btw. A US 1 Cent Coin I have on Hand made out of Copper measures .748 thousand and should work just as well.

Cheers

[Newtire]

No,the tests I saw were in The Double Gun Journal by a guy, Tom Roster maybe? The article was called Finding out for myself. It concerned his ideas about Damascus barrels and pressure testing. He listed a series of loads in 12 and 10 gauge. The pressures were always higher using fold crimps. These were all handloaded. I would have thought, like you, that it would be opposite.

[Newtire]

A roll crimped shell have a longer OAL than a folded crimp. With everything else being equal, this means a more compressed wad = deeper setting = increased pressure.
I strongly doubt the actual force needed to open a crimp has any direct bearing on the pressure, though it may indirectly do so by improving the ignition - like a hard crimp on a pistol load.

With a shotgun shell, the wad is always seated against the powder, unlike some rifle and pistol loads where the bullet seating depth varies the distance from the bullet base to the powder. Deeper seating reduces this distance and that's where the higher pressure comes from no?