please advise re: seating depth

rufous

Well-Known Member
Joined
Jul 17, 2001
Messages
174
Location
Walla Walla, WA
I have been learning some about throat types and dimensions lately. Apparently most if not all factory rifles are chambered with a tapered throat. On a 300 Win Mag the diameter of the throat would be 0.3150” just in front of the end of the neck portion of the chamber. It would then taper down to bore diameter 0.2976” in front of neck (according to the reamer drawing I got from Clymer). A parallel throat should be 0.3085” in diameter and would be however long the gunsmith made it. Some chambering reamers are designed to cut no throat at all and the throat is then cut to whatever length the customer wants with a separate throating reamer. Other parallel-throated reamers have 0.100” or 0.200” or more throat length before the lead.

Anyway my 300 Win Mag was cut with a parallel throat 0.300” long. It was longer than I wanted. I had sent a dummy round to the gunsmith, requesting that he throat it so that the dummy round was just touching the lands. He cut it 0.100” longer than I wanted. The magazine box is 3.600” long and most bullets are touching the lands at a length longer than my magazine box will hold.

I have heard just recently that guns whose chambers are cut with a parallel throat will generally shoot very well with lots of bullet jump to the lands. I did not know this when I got my 300 Win Mag back from the gunsmith and I did not know that it had a parallel throat. So I went about load development under the assumption that the gun would shoot best with the bullets touching the lands or out to about 0.030” off. I got the 220 Sierra Matchking shooting very well at 0.010” off and the 200 Nosler Partition shooting very well at 0.030” off the lands, but both of those loads are too long to fit in my magazine box. I tried many other bullets too including the Barnes 165, 168 and 180 and the Swift 180 Scirocco. None of those shot well at the seating depths I tried but perhaps I should have kept seating deeper and deeper due to the parallel throat.

I have been thinking about having the back side of the action milled out to increase the length of the magazine well. That way I could install a longer magazine box to accommodate the long OAL of my cartridge. But I wonder if I should bother with that, which would cost $150 plus shipping, or if I should just shoot the gun more, trying deeper seating depths. Both options will cost money. Trying deeper seating depths will wear on the throat more. Basically I am wondering if many of you have found that your rifles will shoot tight groups with 0.100” and more bullet jump to the lands if your chamber was cut with a parallel throat or if I should get the action altered for a longer box so I can load long and have minimal jump? Thanks for any advice and experience you can offer, Rufous.
 

Charles A

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Joined
Nov 7, 2001
Messages
480
Location
tennessee
Why dont you just use it as a single shot? I know its not the answer you wanted but I am able to shoot mine singleshot almost as fast as reapeter.
 

Len Backus

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Staff member
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May 2, 2001
Messages
6,643
I am shooting two of mine without using the too-short magazines.
 

rufous

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Joined
Jul 17, 2001
Messages
174
Location
Walla Walla, WA
Len, I am not sure I follow you. Are you saying you just load singly? This rifle is my climb the mountains big game rifle. It is totally unacceptible to me that I not be able to use the magazine. Rufous.
 
G

Guest

Guest
rufous

This is my suggestion,

1) decide which bullet you must hunt with using the mag. functionally

2) Set the shoulder back on the barrel, .090" or whatever it takes to use the mag with the selected bullet, trim the tenon length accordingly, and then run a reamer of the appropriate throat length the needed distance to fix the problem. You may lose some thread off the tenon, but it will most likely be less than 2 threads, and a non issue. If you lengthen the mag well on most of these bolt guns, you get too close to the material that supports the lower recoil lug, and I for one start to get nervous. The backside may not give you the distance you need, measure very carefully, I think the back side will get you more on a Win than a Rem.

[ 01-10-2003: Message edited by: S1 ]
 

rufous

Well-Known Member
Joined
Jul 17, 2001
Messages
174
Location
Walla Walla, WA
Thanks for the replies and suggestions. If I do lengthen the magazine box it would be from the back end. The guy who makes these boxes that Brownells sells would do the machining. He said he can give me about 0.2" more room. But it is looking like I may not need to. I was out shooting the Swift 180 Scirocco and trying deeper seating depths. That bullet is touching the lands at about 3.65" overall length. I tried a couple days ago a seating depth of 3.625, 3.59 and 3.35". At 3.35" and 76 grains of RL22 I got a 3 shot group of 1.14". Then today I tried 75.5 grains of RL22. A seating depth of 3.36" gave a group of 1.55". Seating depth of 3.37" gave 1.14" and seat depth of 3.38" gave a group of 0.79". Then a seat depth of 3.39" gave a group of 0.63". Finally I tried 75 grains with a seating depth of 3.38" and got a group of 0.43". So now I need to try 75 grains and 3.39" since that seating depth was better than 3.38" with 75.5 grains. Anyway it looks like this gun will shoot with lots of jump to the lands (0.260"). I guess a parallel throat will allow a rifle to shoot tight groups with lots of jump to the lands. Rufous.
 

sniper2

Well-Known Member
Joined
Jan 10, 2003
Messages
948
Location
centre,alabama
i HAVE SEVERAL RIFLES WITH 3/8 AND UP TO
1/2 FREE BORE AND THEY ARE SHOOTERS AND i DO NOT HAVE TO USE THEM AS SINGLE SHOTS. IN SOME EVENTS THE DEEPER SEATING WILL WORK WELL. FOR INSTANCE I TRIED SOME LOST RIVER
BULLETS AND SEATED THEM OUT QUITE A BIT AND THE GROUPS WERE HORRID.PROMPLY CALLED THE GUY AT LOST RIVER BALLISTICS TO ASK FOR SUGGESTION AND HE SUGGESTED A MUCH DEEPER
SEATING TO START AT. LO AND BEHOLD THE NEXT GROUP WAS TWO IN ONE HOLE AND THE NEXT ONE ABOUT 1/8 OR SO FROM THEM, I WAS VERY SUPRISED, BUT THIS HAS WORKED ON SOME OTHER BULLETS. GOOD LUCK AND GOOD SHOOTING
 

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