I have had a problem with a rebarrel job on a 7mm saum remington model 7. I was loaned a reamer that supposedly was designed to allow bullets to be seated to the lands and feed out of the magazine. This wasn't the case the the reamer did not have the anticipated specs and the bullets have to be seated about 160 thou longer than mag length to reach the lands.
As I see it if I want to achieve my original aim I either get my barrel pulled and possibly screwed onto a rem M700 long action and use the existing throat.
Or get a reamer designed that will achieve the original aim, and get the chamber cleaned up and the barrel set back.
This has led me to do some reading about reamer specs.
My laymans's understanding at present is that in there are three important dimensions in throat design,
The freebore or section ahead of the neck before the taper of the rifling starts (bore of the barrel with the rifling reamed away).
The angle on the taper of the rifling that starts at the end of the freebore.
The diameter of the the freebore section. My understanding is that the diameter of the barrel in the free bore phase of the throat should ideally be very close to the diameter between the grooves of the rifling.
Now I am trying to gain an understanding of the effects changing the various dimensions has.
So more freebore = greater powder capacity and higher velocity
Shorter or no freebore = higher pressure is reached with lower powder charge and velocity is lower.
Factory rifles usually have a fair bit of freebore as a window of safety to avoid pressure related issues. Some see this as factory-fitted throat erosion. Others see this as no problem there are plenty of accurate weatherby magnums floating around with huge freebore. Perhaps not bench-rest accurate but still pretty accurate.
As far as the the angle that the rifling starts I can see that a small angle would be preferable when using VLD bullets (In my case intend to use 162 AMAX and accubonds) for a smooth enrty into the bore. In two throats with identical freebore bullets will have to be seated out further to touch the rifling in throats with a small entry angle.
Does freebore have any influence on accuracy? or ease of finding an accurate load? In a calibre like 7 Saum is freebore even required and if so what would be a healthy minimum?
What influence does the entry angle have? what angle would you recommend for the bullets I intend to use (Amax, accubond). Does twist rate factor in here anywhere?
In the general sphere of throat design are there other important considerations? In my case where I am looking at designing what I would guess to be a very short throat am I simply inviting trouble?
Thinking about this has also made me wonder about throat erosion. I always thought this merely involved loss of rifling and hence seating length must be increased to reach the lands. Does the diameter of the freebore section of throat also increase or does this remain relatively constant? Do any parameters of throat design such as overall throat length or rifling lead-in-angle influence throat wear?
As I see it if I want to achieve my original aim I either get my barrel pulled and possibly screwed onto a rem M700 long action and use the existing throat.
Or get a reamer designed that will achieve the original aim, and get the chamber cleaned up and the barrel set back.
This has led me to do some reading about reamer specs.
My laymans's understanding at present is that in there are three important dimensions in throat design,
The freebore or section ahead of the neck before the taper of the rifling starts (bore of the barrel with the rifling reamed away).
The angle on the taper of the rifling that starts at the end of the freebore.
The diameter of the the freebore section. My understanding is that the diameter of the barrel in the free bore phase of the throat should ideally be very close to the diameter between the grooves of the rifling.
Now I am trying to gain an understanding of the effects changing the various dimensions has.
So more freebore = greater powder capacity and higher velocity
Shorter or no freebore = higher pressure is reached with lower powder charge and velocity is lower.
Factory rifles usually have a fair bit of freebore as a window of safety to avoid pressure related issues. Some see this as factory-fitted throat erosion. Others see this as no problem there are plenty of accurate weatherby magnums floating around with huge freebore. Perhaps not bench-rest accurate but still pretty accurate.
As far as the the angle that the rifling starts I can see that a small angle would be preferable when using VLD bullets (In my case intend to use 162 AMAX and accubonds) for a smooth enrty into the bore. In two throats with identical freebore bullets will have to be seated out further to touch the rifling in throats with a small entry angle.
Does freebore have any influence on accuracy? or ease of finding an accurate load? In a calibre like 7 Saum is freebore even required and if so what would be a healthy minimum?
What influence does the entry angle have? what angle would you recommend for the bullets I intend to use (Amax, accubond). Does twist rate factor in here anywhere?
In the general sphere of throat design are there other important considerations? In my case where I am looking at designing what I would guess to be a very short throat am I simply inviting trouble?
Thinking about this has also made me wonder about throat erosion. I always thought this merely involved loss of rifling and hence seating length must be increased to reach the lands. Does the diameter of the freebore section of throat also increase or does this remain relatively constant? Do any parameters of throat design such as overall throat length or rifling lead-in-angle influence throat wear?
