Regarding accuracy, the leade is most important and BR guys determied 11/2 degeree per side is optimum and tight, .3085- .309" dia. Please note earlier posting mentioning tangent ogive and Obermeyer (2.5 deg per side).
308 Chamber?
- Thread starter TrxR
- Start date
What is a good reamer if you are shooting from AI magazines? The bisley sounds perfect for what I will be shooting (155scenar, 168 and 175SMK), but will the OAL limit from the mag make it difficult to find the "sweet spot" WRT the lands?
I would look at this sight for the best reamers I have used and other friends, when you order custom reamer the reamer Matches the spec sheet
not so with some others.
http://www.jgstools.com/
2010 JGS 308 Match
308 Win
308 US Palma
308 Obermeyer
not so with some others.
http://www.jgstools.com/
2010 JGS 308 Match
308 Win
308 US Palma
308 Obermeyer
Good thread with great info, but I'm just curious is there some written data report we can look at that shows how much increase in accuracy there's been with tuning the chamber just right for each bullet and competition class and how much gain there was over plain Saami chamber dimensions?
rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
No. There won't be any directly measurable accuracy increase.
The reason for custom chambering is to improve the "handle-a-bility" of things. For example, goals you might be trying to accomplish with a customized chamber:
- brass stretches a bit less on firing (makes resizing easier, and brass last longer)
- put the bullet at a certain place in the neck (so body-boattail junction is ahead of the neck-shoulder junction)
- put the bullet at a certain location so that a particular bullet will fit that rifle's magazine
- maybe you have a theory that a certain leade angle has a longer accuracy life, or is less sensitive to bullet seating depth
- maybe you have to make the chamber of your rifle fit a certain kind of "issue ammunition" (in the old days of TR shooting, that would be NATO military ball ammo; nowadays you might want your chamber to fit Federal 168 or 175 Match ammo, i.e. magazine-length-loaded Sierra 168MK or 175MK)
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
As Daniel posted, there is no benefit in accuracy for using 1 chamber over another these days because we fit the ammo to what is there and we have the tools to adjust our cases and bullets for a perfect fit.
Adding more info to the above:
From the base of the case to the base of the neck, these dimensions are specified by SAAMI AND all competition bodies. This is the DISPLACEMENT of the chamber and many competitors would love to tweak this if they could. Any chamber that is legal to be used as a 308win chamber will vary no more then a few thou here and there so there is likely more variation in the MAKING of the reamer then what is on a sheet of paper
The neck area (top of shoulder forward) only has to be sized large enough to allow the bullet to release properly under the conditions the rifle will be used. By the time the neck moves 1 thou during the firing process, that bullet is on the move. The rest of the expansion allows for an orderly release. More clearance will not hurt, except maybe allow for more neck splits. Too little WILL cause big problems in the form of pressure spikes.
"tight" or close tolerance chambers are only recommended when the rifle will not see a dirty environment and when the reloader will control the thickness of the case neck.
The final area that is played is the throat/leade. here the goal is to allow a particular class or type of bullet to be seated so that max capacity can be achieved vs other set up needs (mag or single feed, etc). The relative location of the ogive to the lands is really arbitrary and can be dealt with during the loading process.
Leade angles are also of little relevance as the actual difference in steel is so minute vs how this area will change as rds are fired - throats do not stay the same.
So a "custom" chamber really is about matching the ammo you want to shoot vs how the ammo will be fed. The accuracy is in the installation of that barrel, quality of the barrel, bedding of the action, ammo tuning of an accurate bullet - The chamber is merely the space that allows this to happen
Jerry
Adding more info to the above:
From the base of the case to the base of the neck, these dimensions are specified by SAAMI AND all competition bodies. This is the DISPLACEMENT of the chamber and many competitors would love to tweak this if they could. Any chamber that is legal to be used as a 308win chamber will vary no more then a few thou here and there so there is likely more variation in the MAKING of the reamer then what is on a sheet of paper
The neck area (top of shoulder forward) only has to be sized large enough to allow the bullet to release properly under the conditions the rifle will be used. By the time the neck moves 1 thou during the firing process, that bullet is on the move. The rest of the expansion allows for an orderly release. More clearance will not hurt, except maybe allow for more neck splits. Too little WILL cause big problems in the form of pressure spikes.
"tight" or close tolerance chambers are only recommended when the rifle will not see a dirty environment and when the reloader will control the thickness of the case neck.
The final area that is played is the throat/leade. here the goal is to allow a particular class or type of bullet to be seated so that max capacity can be achieved vs other set up needs (mag or single feed, etc). The relative location of the ogive to the lands is really arbitrary and can be dealt with during the loading process.
Leade angles are also of little relevance as the actual difference in steel is so minute vs how this area will change as rds are fired - throats do not stay the same.
So a "custom" chamber really is about matching the ammo you want to shoot vs how the ammo will be fed. The accuracy is in the installation of that barrel, quality of the barrel, bedding of the action, ammo tuning of an accurate bullet - The chamber is merely the space that allows this to happen
Jerry
Precision Shooting magazine did a survey asking this question on .308 Win chamber design and it was unanimous from all top gunsmiths that a tighter, .3085" leade aids accuracy. Just asked this of a top gunsmith not in this country who repsonded that a3085 " leade aids accuracy. There is a reason all match reamerss use a tighter spec. He wasn't convinced the 11/2 degree is always optimum and an earlier posts spoke to compatanility depending on ogive.
Regards,
Peter
Regards,
Peter
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
Here is the SAAMI 308 Win chamber dimensions and tolerances. Nominal throat diameter is 0.309" - 310". Some may vary this a schnick but I just don't think 0.0005" to 0.0015" is going to make much of a difference in accuracy. Throat length would be far more important.
Apparently ICFRA has a min spec limit for what is considered a 308W chamber for competition. 0.3085" throat is it. I have never seen anyone measure a chamber so I doubt this is relevant BUT since shooters will fall under International Doping rules in Raton, anything is possible.
Me, I like to stay on the safe side of specs. Bullet vary, fouling can build up at the wrong time, clearance can be a very good thing.
The US Palma team felt a 1 3/4 deg throat was ideal... YIKES. My guess is all that changed 200rds into the bore life but....
If it makes you feel more confident, by all means.
YMMV
Jerry
paperslayer
Regular
- Location
- Chilliwack
Herms likes em tight!
Mystic Precision
CGN Ultra frequent flyer
- Location
- Summerland, BC
Just a thought. If the barrel is stamped 308 HABU, you may not be able to use it in events that specify a 308 Win chamber. yes, it is anal but that decision is left to the match higher ups and some may not have a clue what this chamber is.
I would bring a reamer print and be prepared to educate as needed.
having a 308Win chamber gauge may be a good thing too.
Jerry
