Magazine fed vs benchrest single shot internal ballistic theory

[nksmfamjp]

Single shot: So, traditionally people size throats so that with the bearing surface of the bullet held by the neck and the boa5 tail just in the neck that the bullet just touches the lands.

SAAMI Chambers: Typically these are sized so when rounds are seated to mag length, the standard size bullet is about 0.030" - 0.050" off the lands.

Mag fed long range rifles: I've heard this called by several names, but basically, throats are long like a single shot, but the bullet is seated deep to fit the magazine. When the primer lights the powder, the bullet runs out to the lands, then the powder builds pressure and delivers good accuracy and velocity.

I guess my question is do people have rifles setup like this mag fed long throat option? If so, wha5 is the key to making them shoot? neck tension? runout? Neck turn? Certain bullets? Certain powders?

 

[AZShooter]

Based on my experiences distance to lands, throat diameter and angle of the throat is are important factors especially with VLD style bullets.

I shoot a variety of hunting rifles and one free recoil heavy 6 BR. I have rebarreled all of them over the years with custom barrels and in some cases like the 6mm-284 or 284 win use Remington takeoffs and rechamber. ALL but two of my barrels are chambered using a custom reamer. Those reamers use a few different specifications than SAMMI. IMO the reamer's specs make a difference. The two that shoot well use SAMMI spec are the 6 BR and the 338 RCM and both of these use Berger hybrid bullets. (more on hybrids below).

As to seating bullets, all the bolt action rifles used for hunting get setup as a repeater. The usual approach to creating a load is to seat bullet to fit and feed from magazine. Work up a powder charge then if necessary progressively seat bullet deeper in small increments to fine tune for optimal accuracy. I have found that Berger Hybrid bullets rarely need the seating depth tweaking due to their hybrid ogive design.

I am sure others will add to this thread with their observations.

 

[nksmfamjp]

As to seating bullets, all the bolt action rifles used for hunting get setup as a repeater. The usual approach to creating a load is to seat bullet to fit and feed from magazine.

Ahhhh, so you start at max and shorten to adjust for fine tune seating depth? I was starting at manufacturer data and not sure where to go....I'll start MAX and see how it goes....

 

[woodnut]

It is a trial and error kind of work. I would start short enough to fit a magazine and then extend bullet til you get the desired results you are after. Watch for pressure signs all along the process!
As a side note, I have an AR10 that won't feed the long ogive Berger style bullets. They hit the "wall" below the chamber. If I feed them straight in, all is well.

 

[Elkeater]

Yup as stated by AZshooter I just start at mag length find a charge weight then get shorter from there.

 

[Tiny Tim]

Bullet profile, lead angle, and distance form lands all factor into this. If you want to shoot "mag length" for use as a repeater, that will be your limiting factor. AZshooter has the best advice without changing anything. Sometimes you can get slightly longer mags (internally) or add an extended box (Wyatts or Bainey). Most hybrid type or tangent ogive bullets are less sensitive to seating depth than secant ogive profiles, although I have a 300 WM that shoots 210 gr HVLD's best at .090 off the lands and just fits into the standard mag.

 

[jrock]

Depends on the case and bullet you want to shoot.

Example 1: 338 NM which is a case designed to seat 300 grain bullet like your single shot approach and still fit in a CIP length magazine. It works very well out of the DBM.

Example 2: 7 SAUM case with a 168 grain berger VLD will seat the same as your single shot approach and still fit in a DBM. The throat would need to be cut to match and then you get the same thing.
If the 180 grain VLDs were used in a DBM, the bullet would need to be seated deeper.

 

[nksmfamjp]

I have a 300 WSM. To touch the lands with my bullet, I think OAL has to be 3.14", but 2.95 is max mag length...

 

[jrock]

Long freebore is part of the Weatherby design for speed. It is also part of the 204 Ruger for speed. I have several of them. One 204 is on an AR platform, so I'm restricted on length. The other is in a short action and I seat out to near lands. the second I can add 0.3 grains more powder before hitting pressure and I get about 150 fps more speed. The increase is partly due to 2" longer barrel (~50 fps) and more powder. Beyond that, the internal ballistics are beyond my full understanding. I don't like being restricted by mag lengths.