Jumping/jamming - why does it affect accuracy?

Post by **AlanF** » Mon Sep 03, 2012 12:39 am

We've had a few very experienced reloaders comment on bullet runout on another thread. I'd like to get some opinions on the subject of jamming and jumping. Why is it that accuracy usually varies depending on whether the bullet is jumped or jammed? Obviously pressure and hence velocity will vary, but there must be other things happening - can anyone give us some insight?

Alan

DenisA · Post by **DenisA** » Mon Sep 03, 2012 12:41 pm

Just to add input, certainly not trying to "tell" anyone anything.

To my understanding.

1. Most obvious, jamming aligns the bullet in the rifling. Encouraging concentricty to the bore.

2. In a jumped situation, I imagine that if the bullets not concentric to the bore, when it engages the rifling under power it may cause deeper or irregular groove marks in one side of the bullet and cause an inbalance or distortion in the bullet.

3. It seems to cause a difference in barrel harmonics which allows a great way of "fine tuning". Its amazing how just a couple of thou difference can give you a consistant vertical or horizontal pattern.

I'll be really interested in this thread.

Good one Alan, up late deep in thought!

johnk · Post by **johnk** » Mon Sep 03, 2012 1:16 pm

Adding to that, Boots Obermeyer postulated that coated bullets (moly, boron etc) performed better when jumped a good handful. He felt that jamming removed the coating in a random manner, resulting in loss of uniformity of launch. I don't feel I've been disadvantaged loading with jump - while I use close tolerance throats.

aaronraad · Post by **aaronraad** » Mon Sep 03, 2012 3:55 pm

DenisA wrote:3. It seems to cause a difference in barrel harmonics which allows a great way of "fine tuning". Its amazing how just a couple of thou difference can give you a consistant vertical or horizontal pattern.

Speaking of harmonics has any played around with the Border Barrels Barrel Vibrations Simulator (say that 10 time fast):

http://www.border-barrels.com/articles/rimfire_accuracy/barrel_vibrations.htm

Produces some interesting effects for the time a bullet spends in the barrel and the centre of gravity to the boreline. A friend has an FEA program I might see if he can replicate the program but with a secant & tangent bulllet starting from jump & jam. There would have to be a masters or phd in that somewhere, but hopefully just a carton of beer. :wink:

ned kelly · Post by **ned kelly** » Mon Sep 03, 2012 9:14 pm

G'Day All,
why bother arguing the toss, try jump, touch and jam and which ever works the best CONSISTENTLY, there's your answer, it aint rocket science, it's wot works!
Cheerio Ned

Post by **AlanF** » Mon Sep 03, 2012 9:24 pm

Ned,

This is a discussion forum for people with too much time on their hands. If we want to have meaningless and futile discussions about trivial things, then we'll do just that!

Alan

ned kelly · Post by **ned kelly** » Mon Sep 03, 2012 9:27 pm

G'Day all,
my humble apologies for interupting your valuable time.......until next time!
Cheerio Ned

Chopper · Post by **Chopper** » Mon Sep 03, 2012 9:30 pm

I Had a try of the hybrid 6mm 105g, last weekend at a comp , had two lots batched , one lot in, and one lot just off, over 300m and 500m I am yet to tell the diff, one lot was just on and the other just off, 10 tho the diff, they went ok at both ranges, maybe the longs will tell more, the price is right,Chop.

aaronraad · Post by **aaronraad** » Mon Sep 03, 2012 9:48 pm

Chopper wrote:I Had a try of the hybrid 6mm 105g, last weekend at a comp , had two lots batched , one lot in, and one lot just off, over 300m and 500m I am yet to tell the diff, one lot was just on and the other just off, 10 tho the diff, they went ok at both ranges, maybe the longs will tell more, the price is right,Chop.

I was under the impression that this is what the hybrids were meant to address, less sensitive to seating depth? Got to be happy with that!