How important is throat diameter for precision

[6BRinNZ]

Hi,
The shooting context is FTR
I have a reamer for Berger 185s with a throat diameter of .3085.
I want to go to the 200 and 208 class bullets and I understand the recommendation is to increase the throat diameter to .309 for these heavier bullets.
Can anyone provide experience or thoughts.
I'm worried the extra space may allow more wobble which might be bad for precision.

 

[ShtrRdy]

As far as I know, the diameter of the 200/208 grain bullet is the same as the diameter of the 185 grain bullet. I would continue to use 0.3085" for the freebore diameter.

 

[6BRA]

6mm bullets will range from .24320 to .24380. All are not the same.

 

[ShtrRdy]

6mm bullets will range from .24320 to .24380. All are not the same.

Are you talking about the pressure ring?

 

[6BRA]

Are you talking about the pressure ring?

yes

 

[DJSBRS]

Just remember, the throat diameter is where the carbon ring starts with the first shot to foul the barrel. The smaller diameter the quicker the the throat starts closing up with fouling and eventually a carbon ring begins provided you don't stay on top of your proper cleaning methodology, The same as in front of the brass neck which is shorter than the neck chamber.

I prefer to order my freebore diameter reamers with a .001 over standard bullet size.

DJ

DJ's Brass Service

 

[Will Henry]

As a short range BR shooter, I never even saw a carbon ring on one of my rifles. Later on, as a "F" class shooter, I would shoot all day without cleaning (60 shots or so) but still no ring. I have always liked a throat as close to bullet diameter (ignoring the pressure ring; it'll swage down OK) as possible. So I generally make a 30 cal .3085. This works great until you get some bullets which are .3087. My 6mm throater, which I always considered just fine, is .2433. I once built a 303 British rifle, just for fun. Sierra was making their 174 gr MK. I bought a box of bullets which measured .3112. I ordered the reamer with a .3115 diameter throat. The rifle shot great. It would typically shoot ten shots into an inch at 300m. I even came in second at one "F" class shoot with it, beating a whole bunch of more conventional rifles. Anyway, a year later, I bought another batch of bullets and the rifle wasn't shooting as well as it had been. I measured them and they were .3117. They were sticking in the throat and getting pushed back into the case. I called Sierra about it. The guy I talked to seemed to doubt my ability to read a mike. Ultimately, I ground off the throat on my chambering reamer and cut throats separately with reamers from 3115 to 313.
As far as what is best as far as throat configuration is concerned, I am less certain today than I was 40 years ago. I still think a throat which is close to bullet diameter is best but I have had good results with a long taper as well. WH

 

[DaveTooley]

A modestly larger freebore will perform consistently better barrel to barrel. That's assuming a concentric chamber, which is key.. I've had manufactures want my 308 design. I laugh. I have used nothing but SAAMI 308 reamers with a .310 FB for many decades. +.001 to +.0015 over doesn't hurt anything, only helps.

 

[std7mag]

Not to derail, but would the lead angle matter more or less than the diameter of the throat??

 

[Bill Norris]

Not to derail, but would the lead angle matter more or less than the diameter of the throat??

I've heard and read that many have said the shallow lead angle reamer is much better than the steep angles. Tim Sellars says it decreases the rate of throat wear along with other factors he discusses with Erik Cortina on "Believe the Target" podcast.

https://www.youtube.com/results?search_query=believe+the+target+tim+sellars

 

[gunsandgunsmithing]

A smooth and large/longer transition creates a more efficient heat sink, every time. Same as a bigger bbl. I fail to see his logic with that but will stop with that. Other than to say, I'm currently working essentially the other direction with reamer design. We'll see but early results are encouraging. More specifically, erosion is nil, not even wearing away tool marks or even close, yet AND carbon and the carbon ring is much reduced if not eliminated...fwiw. Still too early but ask yourself why so much less carbon and no carbon ring in that area. I do think there is a correlation. Why does carbon normally deposit and build up at this area in the first place? Hmmm.

 

[Bob3700]

Help me out here, do boattail bullets have a pressure ring at the base?
It was my understanding that only flat base bullets have a pressure rings.
The short range BR group are aware of the pressure rings but they typically Shoot flat base bullets. I would venture that the majority of MR and LR shooters are focused on boat tail bullets.

 

[Jud96]

I’m no expert and have minimal experience with different reamer designs, but any custom reamers I’ve ordered have had a +.001-.0015 over bullet freebore diameter. I feel it can’t hurt anything and reduces the chance of a fatter bullet getting swaged in the freebore and more room for fouling to go.

 

[6BRA]

Help me out here, do boattail bullets have a pressure ring at the base?
It was my understanding that only flat base bullets have a pressure rings.
The short range BR group are aware of the pressure rings but they typically Shoot flat base bullets. I would venture that the majority of MR and LR shooters are focused on boat tail bullets.

See pics

 

[Bob3700]

See pics

Can you explain the pictures? It appears that they are different boattails of bullets from different manufacturers. It also appears that you are not measuring the end of the boattail, but the bottom of the bearing surface diameter. I would expect slightly different Bearing Surface OD dimensions between say, Sierra vs Berger vs Lapua.
My experience has been that Berger 105's are slightly smaller than 107 SMKs by a few tenths. But that OD difference does not constitute what I was taught was a pressure ring.
When I shot benchrest, you could take a flat based bullet and measure the diameter below the ogive and above the base. Next take a measurement at the very base of the bullet and that dimension would be slightly larger by a few tenths. That slight bulge at the end of the flat base was the pressure ring. My understanding was that when the bullet was swaged into the dies, that step caused the base of the bullet to increase a few tenths, thus the pressure ring. The boattail bullets, due to the actual process of forming the boattail, did not have this pressure ring.
No argument here, just trying to understand if we are talking about the same issue?

 

[WSnyder]

I have a reamer for Berger 185s with a throat diameter of .3085.

Who made the reamer? Some almost always cut a bit oversized (PTG) and others cut on size or very close to on size.

 

[memilanuk]

Hi,
The shooting context is FTR
I have a reamer for Berger 185s with a throat diameter of .3085.
I want to go to the 200 and 208 class bullets and I understand the recommendation is to increase the throat diameter to .309 for these heavier bullets.
Can anyone provide experience or thoughts.
I'm worried the extra space may allow more wobble which might be bad for precision.

Recommendation from whom?

The 2013 US FTR reamer spec has a freebore diameter of 0.3085", and has taken more gold than you can count, both individual and team. It was originally 'designed' around the Berger 185 Juggernauts, and has been used since for just about every 200-ish gn match bullet out there - 200 Hybrid, then the 20X, then the 208.

Pretty sure messing with the freebore diameter is fairly low on the list of things you need to be worrying about for success.

 

[6BRA]

Can you explain the pictures? It appears that they are different boattails of bullets from different manufacturers. It also appears that you are not measuring the end of the boattail, but the bottom of the bearing surface diameter. I would expect slightly different Bearing Surface OD dimensions between say, Sierra vs Berger vs Lapua.
My experience has been that Berger 105's are slightly smaller than 107 SMKs by a few tenths. But that OD difference does not constitute what I was taught was a pressure ring.
When I shot benchrest, you could take a flat based bullet and measure the diameter below the ogive and above the base. Next take a measurement at the very base of the bullet and that dimension would be slightly larger by a few tenths. That slight bulge at the end of the flat base was the pressure ring. My understanding was that when the bullet was swaged into the dies, that step caused the base of the bullet to increase a few tenths, thus the pressure ring. The boattail bullets, due to the actual process of forming the boattail, did not have this pressure ring.
No argument here, just trying to understand if we are talking about the same issue?

Correct 3 different bullets and 3 different manufacturers. All boat tails and all bullets are suspended being held by micrometer. They are being measured at base of bearing boattail junction .That oversize area regardless of what you want to call it, has to push through . I would think this results in pressure variances. If I move up on bearing surface toward nose that figure will get closer to .24300.

 

[Wild Bill IV]

A smooth and large/longer transition creates a more efficient heat sink, every time. Same as a bigger bbl. I fail to see his logic with that but will stop with that. Other than to say, I'm currently working essentially the other direction with reamer design. We'll see but early results are encouraging. More specifically, erosion is nil, not even wearing away tool marks or even close, yet AND carbon and the carbon ring is much reduced if not eliminated...fwiw. Still too early but ask yourself why so much less carbon and no carbon ring in that area. I do think there is a correlation. Why does carbon normally deposit and build up at this area in the first place? Hmmm.

You're on the right track! Increase laminar flow, reduce turbulent flow! Simple air and fluid flow dynamics. Remember, there is friction at the surfaces even with gas motion. Especially with hot gases! Always remember that Friction adds more Heat! Friction can also cause turbulent flow! Yes, heavy bbls absorb more heat than thin. But, at a price! The thicker material takes longer to transport that heat and dump it into it's surroundings (Air)! It is based on thermo conductivity (one of several Physical Properties) of the steel alloy of the bbl.

Aim small, hit BIG!!!!!
BILL

 

[FrankG]

As far as I know, the diameter of the 200/208 grain bullet is the same as the diameter of the 185 grain bullet. I would continue to use 0.3085" for the freebore diameter.

You sure about that on bullet diameter? Sorry but even in the same lot I’ve seen bullet diameters vary greatly!

Even lot to lot I’ve seen bullet diameters vary .0005”. As the bullet dies wear the diameters will change. It’s up to the bullet maker on the tolerances they hold and when they replace the dies etc… and not to mention a little more lube on the jacket or cores or jacket lots and hardness etc… will have an effect as well.