A Barrels "preferences" for a Jump or Jam

ecomeat · Post by **ecomeat** » Sat Jul 20, 2013 10:09 pm

People talk about the different characteristics of different barrels with different projectiles......some like a 0.005" Jam, some like a 0.010" jam......some like a jump of 5 or 8 or 10 (or whatever) thousandths of an inch. Vince Northfield from Wild Dog had a 7mm-08 that liked a jump of 0.060"
If a chamber is lengthened by increasing the distance to the start of the lands by say 0.050", or 0.075", ....................hopefully a number of people reading this will have rechambered for different reasons........ will a barrel normally retain its preference for a given projectile, or does it automatically mean starting fresh to discover its preferences for a jump or a jam ??

So by default, if the answer is "its effectively a new barrel.......you have to re-learn its preferences (re Jump or Jam)", then that would automatically mean that its the shape (cut/style ??) of the beginning of the lands that determines whether a barrel shoots a given projectile best with a jump or a jam ??
My own specific case that i am seeking help with is a 284 Win barrel with Berger 180 VLDs. Most people apparently get the 180 VLD to work with a small Jam, as i did with my first barrel.
But playing around with a new barrel for a couple of days, by far my best vertical at 500 yards,..........3/4 "...... and by far the best groups at 300 yards, are both with a jump of 0.050".
Trouble is, with my current chamber, and a slower new lot of powder and tight new barrel meaning more powder, its virtually a compressed load, and the boat-tail/shank junction getting way too close to the donut area.
Hence the Question re a barrels "preferences" if the leade/lands are altered .
Rgds
Tony

IanP · Post by **IanP** » Sat Jul 20, 2013 10:51 pm

Tony, your question about bullet seating depth for VLDs is best answered by Berger who have an indepth article on how to do it right.

Read it here: http://www.bergerbullets.com/getting-th ... our-rifle/

I hear a lot of talk about not seating bullets at the "donut area" of the case shoulder/neck junction. My favourite design ask in getting my reamers manufactured for a specific bullet (any long range bullet) is that they are cut to suit the bullet's bearing surface finishing flush with the shoulder/neck junction. Thats unless I have a case with limited capacity that requires the bullet seated out further with a longer throat to enable this other seating option/position.

If you are using a straight 284W case thats unmodified then there is little possibility of a donut ever forming. If using Lapua 6.5x284 then you will encounter the donut forming because in expanding the neck upto 7mm you have moved thicker shoulder brass up into the neck. Its easy fixed by inside reaming, (Wilson or Forster make reamers for this) or by neck turning back into the shoulder. Once this has been done then you can sit the bullet anywhere you like without problems.

Unmodified brass does not require reaming or turning and you can sit your bullet at the junction without fear.

Hope this helps!

Ian

Brad Y · Post by **Brad Y** » Sat Jul 20, 2013 11:05 pm

Tony here are my experiences.

I shot last years queens with a 6.5x47L and 130gr hunting berger vlds. Had 3 weeks to do load development. With the help of Matt P I started with 37.5gr 2208 that seemed to shoot well in quite a few guns. I loaded a heap of rounds. Jumped 80 thou, 40 thou and jammed 10 thou. I had shot a 22Br years ago with 90gr bergers and they worked bet jammed. And I found HBC's to work best jammed in the 308 as well. So going with the jam logic for vld shaped bullets I thought the 10 thou jam load would be the best. Bugger me dead if 40 thou jump didnt shoot really well and end up winning me the queens.

When doing the load work for my 260 improved and 140 vld's I went straight to a 10 thou jam and found a mild load that shot very well.

What I believe is the VLD shaped bullets do work well jammed 10 thou. But like everything you need to tune the powder charge to suit. Pressure is going to be up with a jammed bullet compared to a jumped one. So if your running say (and I dont know 284 loads yet) 50gr of powder with a jumped bullet, then you may need to run 49.5 or so to get same velocity and node with the bullet jammed. Or you could look at running a higher node and look at say 50.5 I confirmed this in the 260 with 43.7gr 2209 shooting well with jumped hybrids and 43.2gr shooting same POI with 140 vld's jammed.

So if I had more time before last years queens I probably could have worked up a jammed load with VLD's with what I guess to be about 37gr 2208. Of course if you get really good accuracy with jumped bullets I reckon its worth using as the last thing you would want to do is have to unload in a cease fire and have a bullet stuck in the rifling and an action full of powder. I havent seen it happen but you hear the odd story about it.

End of the day if your gun is shooting well with 50 thou jump then I wouldnt worry too much. You could always look at going to another bullet with less bearing surface so your not in any potential donut.

RDavies · Post by **RDavies** » Sun Jul 21, 2013 12:09 am

I have only rechambered a barrel to a different calibre 8 times. (actually 2 others, but cant remember preferences)This is what I found
#1 22/6mm rem imp to 22BRX. Both preferred jam
#2 22/6mm Rem imp to 22BR. Preferred jam, then preferred jump with one bullet (70gn FB), jam with the other (75gn VLD)
6BR to 6 Dasher. Preferred jam, then preferred jump (103 copperhead)
#1 6x47 imp to 6BR. Preferred jam, then preferred jump.
#2 6x47 imp to 6 Dasher preferred jam then preferred jump
#1 7mm/300wsm to 284. Preferred jam then preferred jump
#2 7mm/300wsm to 284. preferred .090" jump with hybrids and .015" with VLDs, then preferred jam with hybrids.
#3 7mm/300wsm to 284. Preferred .020" jam, then preferred .005 jam.

I usually used the same bullet, but always different powders. I think all of my reamers have the same 2 degree leade angle.

bruce moulds · Post by **bruce moulds** » Sun Jul 21, 2013 6:24 am

the old story.
let the barrel do the talking.
donuts can form with time, particularly with cases where the shoulder brass is thicker than the neck brass.
keep safe,
bruce.

ecomeat · Post by **ecomeat** » Sun Jul 21, 2013 7:13 am

RDavies wrote:I have only rechambered a barrel to a different calibre 8 times. (actually 2 others, but cant remember preferences)This is what I found
#1 22/6mm rem imp to 22BRX....................

Thats exactly the sort of data i was hoping to get. Thanks again Rod for sharing specific and detailed information thats backed by experience with a number of barrels.
It seems certain from your experience that if i get my 'smith to run the reamer in enough to just move the start lands forward 0.050" or a fraction more, then my barrel WILL behave differently.
Thank you also to Ian, Brad and Bruce for your comments.
My testing on seating depth was done with 5 different depths, shooting 5 shot groups at/of each depth but done "round Robin" style at 5 different targets and firing 5 x foulers at the start, to warm the barrel a bit and sight in. Its not exactly as Berger suggests, but close enough to show fairly dramatic results in favour of the 0.050" jump.
My first 284 barrel, as well as three 6 x 47 Lapua barrels all had good accurate loads developed with VLDs rather easily by simply jamming 0.008 to 0.010 and then working on powder charge to vary velocity.
One of those three was the one that died a very premature and expensive death after I wrecked it with Redding Imperial Seating Media by not getting rid of 100% of the neck turning lube.
The first 6 x 47 Lap barrel also switched over to 105 gr Hybrids from VLDs perfectly without changing anything.
I would love to hear the experiences of anyone who rechambered but kept the same calibre and powder if there is anyone "out there" !
Rgds
Tony

Matt P · Post by **Matt P** » Sun Jul 21, 2013 8:57 am

I have rechambered a few for myself and a couple of others, and have found that in all cases we just carried on using the same load and jam but in all cases it has been with VLD's which "generally" prefer jammed anyway.

Regards
Matt P

RDavies · Post by **RDavies** » Sun Jul 21, 2013 9:27 am

Following on from what MattP says. (He chambers my barrels and repairs my throating jobs)

I listed the barrels which were chambered to another calibre.

I have had a few barrels rechambered to the SAME cartridge with the same reamer and powder. In these barrels, they preferred similar seating depth and loads. If I had let a barrel go for too long on its first chamber, I would have to bump loads up and seat the bullets out further to chase the lands. Once they were rechambered I would reduce the seating depth and powder load to what it liked before, when it was newish.
Same if I had a barrel throated out. I would have to increase the load slightly, but if it liked .015 jam before, it would like .015 jam after throating.

This is just one of the reasons I like to rechamber old barrels to the same calibre. Powder, primer, seating depth, cleaning, fouling preferences are all very similar to what it used to like.

Paul Janzso · Post by **Paul Janzso** » Sun Jul 21, 2013 6:17 pm

I also have had barrels re chambered. The first one, Rod will remember from a queens at Canberra. Rod bore scoped it and I had to have approx 4 inches cut off it, my first 300 WSM. I was stuck with the same dilemma where to start but I found it liked the same powder charge and seating depth 15 thou jam with 210 VLDs. That re chamber brought the barrel back to as new accuracy.
I also have another 30 cal barrel chambered in, now, 300 Win Mag. It started life as a 30 BOO BOO then 300 WSM now it's current 300 Win Mag chambering. My load testing with the new chamber has proven it likes the 15 thou jam with 190 and 210 gn VLD'S.
I have found by accident, the 190 VLD'S don't mind a jump as my first 30 cal barrel started giving elevation and when I checked the throat, it had eroded 50 thou. I adjusted the seating depth and all is well.
I would suggest if you find a load that likes a jump experiment a bit more and see how far you can jump your load before your accuracy goes west. Then just after a OPM etc after you have cleaned your barrel, check your throat erosion and adjust your seating depth accordingly to preserve accuracy.
This is what I do as I have been cought before.
Cheers
Paul

Quick · Post by **Quick** » Sun Jul 21, 2013 6:59 pm

Gents,

Slightly off topic but if you get the neck reamed out (in my case 266 to 273) can that change what a barrel likes?

Cheers.

Paul Janzso · Post by **Paul Janzso** » Sun Jul 21, 2013 7:14 pm

All I can say in answer to your question is.
All my reamers are match except for the Win Mag reamer, it is a standard reamer. The Win Mag chambering still likes the jam so you will just have to load and try for yourself.
Cheers
Paul

Matt P · Post by **Matt P** » Sun Jul 21, 2013 8:22 pm

Quick
I rechambered my tightneck 284 to a no turn, load stayed the same, then rechambered again to 280AI, still likes a jam.
Matt P

Paul Janzso · Post by **Paul Janzso** » Sun Jul 21, 2013 9:23 pm

There you go Quick.
Mat has done the home work.

Quick · Post by **Quick** » Sun Jul 21, 2013 10:23 pm

Cheers guys. I can safely at I will never have a tight neck rifle again!!!

Tim N · Post by **Tim N** » Mon Jul 22, 2013 8:13 am

Matt,
With the no turn chamber do you still turn a minimum off the necks to make them uniform or leave the brass as is?