Gentlemen;
I am, inter alia, a Springbuck hunter from South Africa and has followed with interest your discussions on the calculation of the distance between bullet and rifling.
I use a Sako Varmint in .270 Win (120gr culling bullets) or a Tikka .223 Rem (55gr Sierra Gameking) for distances up to 350 metres (sorry, we don't do yards in Africa).
The problem presented is one of aestethics in that, should I increase the total cartridge lenght (82,2 mm for the .270) to be the suggested 1 mm away from the rifling, the case neck clearly shows the seating depth and attracts curious looks on the benchrest range . With the bullet seated deeper, the case neck is obviously strectched uniformly all the way and my carefully tumbled cases can be displayed with pride. Does this issue really make the helluva difference you guys are advocating? I have only started lenghtening the cases recently and the extra 2,5 mm has not impacted on accuracy. As I do not have a chrony (yes, Christmas only), what is the effect on velocity?

first of all welcome to the forums , it sure is nice to find out there is some South-Africans out here. I have found that bullet seating depth has a great impact on velocity as well as accuracy . My .22-250 ( my springbuck rifle, headshots ) , which I found the perfect load for today , after countless bullets shot through
the rifle this is what I came up with today , with a loading using ( S 341 , 37.5 gr , 50 gr speer , CCI 250 Mag primer seated at 1mm off the lands ) the velocity sped along at 3580 fps , by simply seating the bullet to touch the land the velocity jumped to 3675 fps . As you can see the seating does make a difference with velocity and pressure . As for the grouping with first load , 28 mm CTC , with the bullet touching the lands it grouped a very pleasing 9mm 5 shots at a 100 m . My tests are much more in detail but is too lengthy to discuss , drop me a e-mail if want to know more. But as a rule of thumb . the closer to the lands the more the pressure spikes.

Regards
Rudie

Karoo, use whatever seating depth that shoots best in your rifle, and forget how it looks, AS LONG AS IT WORKS O.K. IN YOUR MAGAZINE!! I have a Ruger No. 1 in 7X57mm that has a very long throat. The ammo I load for it looks very strange, with the bullet seated out so far that it is longer overall than a .30/'06 factory load. So what? It allows me to get more of the slow powder I use into the case!!

The seating depth will have an infinitesimal, if any at all, impact on your muzzle velocities!!

First, my data shows that going from touching the rifling to 2 mm off increases velocity 100 fps in my 7 mag.

Second, if you can see the seat of you bullet depth on your loaded cartridges, I submit that your dies size your necks down to far. This needlessly overworks the brass, and invites unpleasant run-out problems.

In the end, though, who cares what the cartridge looks like, if it shoots! FWIW, Dutch.

Some things to think about.

(1) I've found that the closer I seat the bullet to the lands, the more very slight changes in seating depth can have large effects on pressure and although I'm a very careful loader I like to leave a little "safety space" just in case.

(2) Depending on the recoil of the rifle you should leave somewhere around 2/3's bullet diameter in the neck. That is, if you are shooting a .308 at least 0.205" of the bullet should be in the neck.

I'm a little surprised at some of these replies.

First, I can't imagine someone at this forum said that seating depth has an "infinitesimal" effect on muzzle velocities. Ram that bullet into the lands before you fire it and you can get enormous/dangerous increases in pressure. Certainly velocity increases with that increased pressure more than infinitesimally - to a point. That is not to suggest that you SHOULD do that in order to increase velocity, but rather that velocity will increase in an amount substantially greater than "infinitesimal". Also, I would certainly agree that SHORT of touching the lands, changes in seating depth will result in small, but not "infinitesimal" changes in muzzle velocities.

Next, no one has mentioned the effects of cartridge OAL, throat length, and harmonic tuning. I haven't been here at this forum for a little while; has this concept "fallen from favor"? It remains "in favor" with me, and is the primary tool I use when working up loads.

For Karoo, there are several ways to "tune" your cartridges to your bbl and chamber. One you may be familiar with is the BOSS system employed by Browning and USRAC (AKA Winchester)and developed over fifty years ago by several european military sniper teams. The same effect can be accomplished, albeit with considerably more work, by adjusting the OAL (overall length) of your cartridges, by changinging seating depth.

Most people that use this method, find that the "sweet spot" has the bullet backed off the lands usually somewhere between 0.005" and 0.015" (0.127mm and 0.381mm). It is not uncommon, in my personal experience, to have group sizes shrink by half simply by adjusting seating depth.

Most people I know, start at the lands, back off 0.005" then move back in 0.002" increments until they get to 0.015". AS I said, usually, (but not always) the sweet spot is found before reaching 0.015" back.

Finally, if someone badmouths the looks of my ammo at the range, they can "put up or shut up". It's how it shoots, feeds, and reloads that matters. To me anyway.

Paul

[ 09-12-2002, 20:53: Message edited by: gitano ]