6 replies to this topic

[Sparky]

Hi all

 

I fired a few shells a little while back and I found that my lift was a little bit lacking for my 4 inch shells but seemed fine for 3 inch shells. I used proportionately more for the 4 inch so I'm not 100% sure I understand the problem. My BP is not brilliant and I use a bit more than 10% to compensate, it's granulated through a grater and as I aid lifts 3 inchers fine. The 3 inchers do seem seem to be a better fit.

 

What's more I actually had a little commercial BP for one shell and the lift was adequate but not totally satisfactory. I'm using cardboard tubes for now and even after 8 layers of pasting there is a sizeable gap around the shell. I suspect this does not help and I wondered if anyone has packed out a shell in some way...e.g. a thicker cardboard layer to start just to get the fit better. I've seen some cylinder shells being loaded that literally have to be forced down the tube and I know how pressure plays a huge role in the way BP burns.

 

Would adding a layer of thicker board as a base layer ruin my breaks?

[Atom Fireworks]

So we are talking about cylinder shells? How big is the gap around your current shells when inside the mortar? Closing that gap I.E making your shell fit tighter will certainly improve the performance of your BP with the effects of confinement. Making the gap too tight can be detrimental, it is thought that some of the passing gasses cuts the immediate air in the path of the shell and aids performance slightly. I am not 100% sure on that theory myself however its good practice to have a shell that can slide into a tube under its own weight.

 

The test I used to use when I made shells was to drop the shell down the tube, if it just slams or thuds into the base of the tube then your gap is to large. It should slide down the tube but be cushioned by the compression of the air below that cant escape faster than the shell is compressing, you will know when you have it just right as you dont hear that thud of the shell hitting the base, rather a gently bump if that.

 

Jay

[Sparky]

Hi Jay

 

Nice way of putting the way you test. Mine definitely land with a thud!

 

Mine are spherical paper shells. They measure the usual 3 inch less about an 1/8th inch each side and the tubes are spot on 75mm ID. This just seems to be a less than perfect match and I really don't want or need to be pasting more layers to make up the gap.

[michaelp]

Hi all

 

I fired a few shells a little while back and I found that my lift was a little bit lacking for my 4 inch shells but seemed fine for 3 inch shells. I used proportionately more for the 4 inch so I'm not 100% sure I understand the problem. My BP is not brilliant and I use a bit more than 10% to compensate, it's granulated through a grater and as I aid lifts 3 inchers fine. The 3 inchers do seem seem to be a better fit.

 

What's more I actually had a little commercial BP for one shell and the lift was adequate but not totally satisfactory. I'm using cardboard tubes for now and even after 8 layers of pasting there is a sizeable gap around the shell. I suspect this does not help and I wondered if anyone has packed out a shell in some way...e.g. a thicker cardboard layer to start just to get the fit better. I've seen some cylinder shells being loaded that literally have to be forced down the tube and I know how pressure plays a huge role in the way BP burns.

 

Would adding a layer of thicker board as a base layer ruin my breaks?

 

If lift is just a little bit lacking try 8% Viagra,   sorry couldn't resist

[megabusa]

I've just measured a MLE 3" shell & found it to varies between 68 to 70mm in diameter. How does that compare to yours ?

 

As I understand it, a gap is needed to allow the gases past the shell as it lifts to help 'lubricate' the shell in the tube.

 

How are you making your lift ?

 

How long does it take for a 1 metre length of BP to burn ?  I have a small section of angle Aluminium anout 8mm x 8mm. I fill the trough with BP & have made a set up that times the burn.

Mine is between 0.5 and 1.5 seconds depending on the quality of the charcoal & my patience waiting for the mill !

[starseeker]

I consider my b.p to be pretty fair powder and i would use approx 30 to 35 grams to lift my 4inch shells,for my 4 inch canisters i would use about 45 grams. The ten percent of shell weight guide is only very approximate and will very lightly under lift them.