Throating for extra speed?

C

Curious

New member
I recently built a 6.5x47 with a second hand reamer that had 0.070" free bore. The chamber work was well within 0.0005" in all the important areas and the rifle shoots very well. I have a problem though in that Im getting pressure signs a little too quickly during load development and it is stopping me from reaching my target speed. With a 0.070" free bore the bullet is seated a little deeper into the case than I would have preferred and its got me thinking that is the reason Im seeing premature pressure signs (bolt starting to become stiff to lift) at 37.6gr RL15 with the 123 AMAX.

Im wondering if it is worth dialing the barrel in again and then cutting the throat to allow me to seat the bullets an extra 0.070" out of the case, this would still leave enough bearing surface in the case and a little extra to chase the lands if needed in the future.

My question is do you think this would then allow me to use more powder to gain the extra velocity while staying within safe working pressure?
 
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Increasing the effective case capacity would enable you to achieve a higher velocity:

  • With the appropriate powder
  • Not necessarily as much as you want
 
John, hopefully he is just wanting to increase the throat length with a throating reamer. If he is talking more case length he might as well chamber it 260. I don't think the longer throat will accomplish what he is wanting.
 
Technically, putting in a longer throat will increase the overall case capacity a small amount. That is, if you define overall chamber volume as the bolt face to the point of first resistance.

But as Butch said, we are taking about a small amount.

Why not try a different powder first.
 
Curious said:
My question is do you think this would then allow me to use more powder to gain the extra velocity while staying within safe working pressure?
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you refer to a "target velocity" as if it's a thing.... in this final sentence the only thing keeping me from saying 'yes' is your verbiage........ what exactly is "the extra velocity"???

The entire question, as asked, is not answerable

If I asked you "I've been eating my wheaties and jumping every day but I haven't reached my target jumping height, do you'se think that I could gain the extra height without blowing a nut by adding some rice kernels to my wheaties?" could you answer me?
 
BTW I'm not being facetious, I have 7 different throat lengths in various 6BR's from .000 to .160 and I've throats from .000 to 145 in at least 5 increments in 6X47L's.......I own throating reamers in .22, 6mm, 30, 338 and .510 and spend a lot of time doing exactly what you're asking about.

Back in about '93-'95 I wrote on this forum about hitting 2900fps with 105's in a 6BR and the roof nearly blew of ;)

I did it with a long throat.
 
I know of quite a few 6.5x47 shooters who are getting around 150fps more from the same powder/bullet/primer/barrel length combo.

During the load development I was surprised at how early in the charge weight the bolt started to get stiff, I was expecting at least another 100fps or maybe a bit more. At the end of the day the rifle is very accurate where it is so that is ultimately the most important thing.

The reason I asked the question was because Im not entirely sure I understand the whole pressure thing. Ive read and been told that reducing case capacity increases pressure so I was thinking that the opposite would also apply. If I made a bit more room in the case by seating the bullets 0.070" further out then would I be able to add another 1.0gr-1.5gr of powder while keeping the pressure at a safe level?
 
Curious said:
I know of quite a few 6.5x47 shooters who are getting around 150fps more from the same powder/bullet/primer/barrel length combo.

During the load development I was surprised at how early in the charge weight the bolt started to get stiff, I was expecting at least another 100fps or maybe a bit more. At the end of the day the rifle is very accurate where it is so that is ultimately the most important thing.

The reason I asked the question was because Im not entirely sure I understand the whole pressure thing. Ive read and been told that reducing case capacity increases pressure so I was thinking that the opposite would also apply. If I made a bit more room in the case by seating the bullets 0.070" further out then would I be able to add another 1.0gr-1.5gr of powder while keeping the pressure at a safe level?
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But, is it the same barrel make as well? Two identical barrels made by the same barrel maker probably wouldn't get the same velocity. But, I'd expect them to be closer than barrels from different makers. Increasing the freebore length sounds like what Roy Weatherby did with his cartridge line. He kept increasing the throat length and was able to add more powder because of the longer throat. Chronographs were few and far between so he assumed he was getting more velocity because of using more powder. Turned out not to be so. Was talking with JGS a few weeks ago and they said the original Weatherby throat was 3/4" long. It's now been reduced down to 3/8" long. The reamer I ordered was a .270 Wtby with a .100" long freebore. Will work as long as the owner realizes that he can not shoot factory ammunition in the chamber. If you want it throated longer, throat it longer and see if it does what you want it to do. We have the advantage now days of readily available chronographs and can see what a change actually does do. With target rifles, barrels are expendable and replaced often. If a barrel doesn't do what I want, I wouldn't be married to it and replace it with one that did.
 
I'm thinking Mike is right. Pressure is dependent on several things but all else being equal a pressure difference just about has to be related to the barrel volume...on a micron by micron basis. It just takes one little tight spot to affect a case. "All else being equal" is a rather nebulous term as well.

Rather than have a target velocity, force yourself to have a target accuracy....because "a fun gun is an accurate one."
 
Lower Velocity than anticipated..

Curious said:
I recently built a 6.5x47 with a second hand reamer that had 0.070" free bore. The chamber work was well within 0.0005" in all the important areas and the rifle shoots very well. I have a problem though in that Im getting pressure signs a little too quickly during load development and it is stopping me from reaching my target speed. With a 0.070" free bore the bullet is seated a little deeper into the case than I would have preferred and its got me thinking that is the reason Im seeing premature pressure signs (bolt starting to become stiff to lift) at 37.6gr RL15 with the 123 AMAX.

Im wondering if it is worth dialing the barrel in again and then cutting the throat to allow me to seat the bullets an extra 0.070" out of the case, this would still leave enough bearing surface in the case and a little extra to chase the lands if needed in the future.

My question is do you think this would then allow me to use more powder to gain the extra velocity while staying within safe working pressure?
Click to expand...

There are many factors that can take play in velocity variables: One that you may not have thought about besides throat length and that is bore diameter and land groove ratio. A few years ago I bought a couple dozen barrels from a top Barrel maker and they would develop pressure before achieving desired velocity. I had a conversation with another top barrel maker and he had me slug the barrels. The bores were consistent but the lands were .005" to .006" to wide. The barrel maker replaced a number of barrels for me and others due to rogue employees efforts to get the job done without adequate knowledge and supervision.
I can say the barrel maker went to great lengths to make it right. It has made me clean and slug every barrel I buy before I go to the trouble of chambering it. It also has made me refuse to chamber certain barrel makers barrels that I can not guarantee to shoot for my customers. If I provide my customer with a barrel I guarantee that is will shoot. They however pay for what they get.
 
See Weatherby.

The 'double radius' shoulder was not as important as the long throat.

It removes the pressure rise from engraving on the rifling out from the peak chamber pressure.

The available volume is larger since the bullet has moved and overcome it initial momentum before engraving occurs.
 
Increasing freebore will reduce pressure and the velocity will also drop right along with the pressure. Load back to the same pressure and you will find the velocity will also be the same or very nearly so. Seating the bullet out into the longer throat until the effective freebore is the same will accomplish a slight increase in case capacity. If you are able to increase case capacity by 2% (which is probably about right), you may be able to increase velocity by about 0.5%. I don't think this will be quite what you are hoping for. Regards, Bill.
 
I'm curious as well...

Certainly, I don't know anything (ANYTHING) about gunsmithing but why would you want to change a rifle that "shoots very well"?
 
Wilbur said:
Certainly, I don't know anything (ANYTHING) about gunsmithing but why would you want to change a rifle that "shoots very well"?
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Wilbur, my guess would be that his brass is getting wrecked, and having to make new cases all of the time is a real hassle.

Many of us go through this with our PPC's. We feel like the best agging capability is at a pressure that results in reduced brass life, sometimes as few as 3 to 4 firings. It seems like a good idea at first, but then you have a big match coming up, your brass is shot, and it's Friday night, your tired.......

Well, you get the idea.
 
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Wilbur said:
I've prolly been there more than most. I dearly hate to make new cases!!!!
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Do they still make the swage set set to tighten up primer pockets that have expanded?

I have one hanging around somewhere.

Used it a few times but decided it was not worth the labor.
 
Don't know if that device is still available or not. I don't think I ever owned a set but Jef Fowler sold an entire bazillion used PPC cases once and the primer pockets were all tight when we bought them. I actually tested the cases I bought. One shot later the pockets were rather loose to say the least. Clearly...Jef had one of those things!

We weren't too miffed about this because at that time you were lucky to have brass in any capacity. We did, however, remember the occasion....as you can see...

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Added: I did have a set of dies that would reduce the primer pocket size. In my stupidity, I forgot. Somebody wanted them a long time back and I couldn't find them so I guess I "loaned" them to somebody. Pretty sure I got the die set from Richard Collins....and pretty sure Richard was glad I did.
 
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