Opinions wanted on Cart. OAL?

[corey006]

I have a 7mm Rem Mag Ruger No.1 with quite a long throat.

The barrel is 26 inchs.

I am trying to work up a load with Retumbo and 160gr Accubonds.

I measured the OAL length using the bullet jam into the lands technique and the average came out to 3.520.

Should I start at 3.505 or 3.490?

Will I get more velocity closer to the lands or further away?

I know that pressure will rise sooner the closer I am to the lands.

Anyone have any experience on long throat chambers and if MORE distance to the lands increases velocity?

I am hoping to achieve 2950-3000 fps without pressure signs(of course with MOA accuracy). I know Retumbo would be better with heavier bullets but I wanted to give it a try anyways. I have tried H-1000 in the past but had trouble getting over 2900 fps without getting pressure signs. I also have some RL-22 but haven't tried it yet because of its "rumored" temp sensitivity.

 

[Ol` Joe]

Since the SAAMI max is 3.290" for the 7mm mag ( I know this is only the lgt that fits all factory rifle mags and isn`t cut in stone) I`d go with this lenght. The volume of the chamber/case is enlarged by seating the bullet out, especially an added .2" and this can reduce pressures at ignition and actually reduce the loads overall velocity. The data was likely developed within this COL and the chamber/case volume is a large part of the inital pressure rise.
A long free bore "usually" helps keep pressures rising at a smooth rate. Weatherby uses a long throat to help keep pressures in line in their rifles. It can`t however be counted on to reduce pressure. A shorter jump of the bullet can raise pressure in my very limited experiance, but it also often doesn`t show any difference as long as the bullet still has a bit of movement prior to contacting the lands.

 

[NCsmitty]

It remains to be seen if you can actually achieve your velocity goals with Retumbo.
Your main concern is the COAL at this point and I would say that you should load 10rds of each COAL that you mentioned, with the powder charge of your choice, and try them. I only suggest that you keep the bullets off the lands to avoid a possible pressure spike.
IMO, 160gr bullets are the ideal weight in the big 7, and the Accubonds are a premier slug for big game.
The ability to seat the bullets out is advantageous because it frees up case powder space to help the velocity issue.
The Ruger #1 is a sweet rifle.


NCsmitty

 

[ranger335v]

"I measured the OAL length using the bullet jam into the lands technique and the average came out to 3.520. Should I start at 3.505 or 3.490?"

I don't think it will make a lot of difference. Develop your max, or best shooting charge at the starting distance you chose and then work back in maybe 10 or 15 thou steps until you get best accuracy.

Book OAL is no more a "law" than the book's suggested charges. It's the length the book makers used to develop their data and they try to choose a length short enough to work in most rifles. Your's is not "most", it's an individual so load what it needs and likes. Just use a measure of common sense and all will be well. In over forty five years of doing this I've never paid any attention to book OALs and neither have any of my contemporaries.

 

[corey006]

Sounds like I am going to be buying at least 10 boxes of bullets and 8 lbs of powder....and alot of experimenting next year....too late this year to play around for 2 weeks tweaking...

 

[woods]

Typically a longer jump to the lands will decrease pressure and velocity, all other things being equal. Reference this chart from Barnes written in one of their magazine articles

http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

This is a chart of different calibers and the pressure taken with the closest seating of .025" depth on the left for each caliber and increasing by .025" going to the right in most cases. The pressures generally show to decrease with increased seating depth



Here is what Hornady had to say

http://www.hornady.com/ballistics/internal.php

To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).

When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure.
Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.

You'll have to go to the website to see the illustrations. But the point is that seating deeper decreases pressure and velocity, probably up to a certain point.

Ol Joe is correct in illustrating that it is not really the distance from the lands as much as it is the size of the combustion chamber that determines pressure and velocity. A chamber with a long throat has a larger combustion chamber because of the added volume created when the bullet is pushed to the lands. That movement is essentially pressure free and the pressure doesn't really built up until the bullet is against the lands. That extra volume means that to get the same velocity as a chamber with a short throat, you should be able to use more and slower powders.

But to answer your question, I would seat the Accubond around .025" and look for a good powder load. I would guess that you will not reach book velocities if you stay under book max loads.

A good powder that seems to work well in situations like this for me is IMR7828SSC with a mag primer. I was having problems getting velocity from a 300 Wtby with other powders.

As always start low and work up.

 

[corey006]

Great post woods...TYVM!

Your right this rifle can easily go 2-3 grs over listed book values and still show no pressure signs.

I loaded H-1000 last year until I got to 3050 level and was showing some pressure signs.(sticky extraction and marks on primer).... I backed it off about 1.5 gr to 2950 fps.

Thought I would try Retumbo this year as I read it was a little better than H-1000.