Freebore clearance

[Rich S]

Wondering what folks thoughts are on the correct amount of clearance for freebore. Most of my reamers are 0.0008 to 0.001 over nominal bullet diameter. I’ve had a few recommendations to increase it slightly.

 

[ronemus]

After measuring the bullets I plan to use, I go for 0.0005" over. This is for target rifles, not combat weapons which could use more clearance for field conditions,

 

[LVLAaron]

1 thou over. Measure some bullets. You'll be surprised.

 

[Rich S]

Thanks for the replies. This will be for Benchrest rifles - mostly 6BR variants. I was wondering if there is any downside to going a little over 0.001. Not really sure I understand what takes place from start of ignition until the bullet is in the bore. Other than alignment is there any negative affects.

With most of my 6mm rifles I do load into the lands. Typically 0.006 to 0.010 into the lands from touch.

 

[Will Henry]

I like to have between .0003 and .0005 actual clearance on the diameter. The thing is, bullet diameter does vary a few tenths. I once built a 303 British as an "F" class rifle (sometimes I do some weird stuff). I bought some Sierra 174 MatchKings and miked them at .3112". With this measurement in mind I ordered my reamer with a 3115 Throat. It worked great. I even shot a 200/200 with 14 X on the ISSF target. Five consecutive five shot groups averaged .328. It was a pretty good shooter. The next year, I had to buy some more bullets and the rifle just didn't perform as it had. I finally noticed that a shell which was extracted without firing it had the bullet pushed back into the case. I measured the new bullets and they were .3117, a half-thou bigger. I ended up buying a .312 throater and put the 303 barrel on a #4 Lee Enfield for a hunting rifle.
The moral of the story is, if you cut the throat too tight, it may be very bullet specific. This can be alright, until it isn't. Nonetheless, I cut my 30's to 3085, unless I have a reason to go larger (I have a 309 for that). I have a 3080 reamer which worked great for some custom 168's which were .3078. Maybe the best 308 combo I ever had. But only with those bullets. For me, .0005 over nominal seems to be a pretty good place to be. Usually. WH

 

[ELR LVR]

I was wondering if there is any downside to going a little over 0.001.

Blowby
(Unless it fully engages the rifling and creates a perfectly swaged seal before leaving the neck)
Then there's the fact you're creating a step in the freebore before it steps back down to bore dia.
The bigger the step, say if it were .005" over, thats a good carbon catcher/holder.
If anything, I'd want nothing over bore dia. --- a clean "line fit" with no junctions or steps to keep the bullet straight in line.
But then again, not all bore dia. are perfect, so you would not want the freebore area to act as a
bullet "swager downer" ---- so .001" over is typical.
(Everything getting tighter on down the bore, is better than everything getting looser)

 

[DaveTooley]

.0012" would not hurt anything. I've designed many wildcats over my career. Most with .0005" over FB's. With what I've learned in the last 25 years that was wrong approach. Yes it's about bullet diameter. More importantly it's about barrel groove diameter. Cut vs. buttoned. As a rule cut barrels are closer to being on size. I'm working on a job now that came with a barrel inspection report. Most of the groove diameters are 2-3 tenths over. So there's 2-3 TENTH's ,in capital letters, allowance for misalignment before a portion of a land or lands comes all the way back to the neck.
Not beating up on buttoned barrels, as they shoot just as good as cut but can be oversize. So a 5 tenths over FB cutting a groove diameter that 5-7, maybe more, tenths over. Throw is concentricity issues and you have a problem.
Regardless of size carbon will build up. Simple solution. Clean frequently.

I designed the reamers for USSCOM for the 300 and 338 Norma Mag's. Knowing how those rifles will be treated I went with FB's that were .0012" over. You should have heard howling from those same guys that don't know how to clean a rifle. I also provided barreled actions for bullet testing to Picatinny Arsenal. All the 300 NM barrels shot 1/2 MOA at 1,000 yds.
I quote one gentleman. "It didn't hurt anything"

So larger doesn't hurt anything. It just eliminates the chance of unseen variables to creep in.

And it doesn't make the phone ring.

 

[Alex Wheeler]

Theres a sweet spot. You can go too small and too big. Small seems to cause fliers, too big just gives up a little raw accuracy. You have been given pretty good advice.

 

[X Count]

So what happened to the "bore rider" fad?

 

[Sniperhandle]

So what happened to the "bore rider" fad?

People got "Bored" with it.

 

[tobnpr]

JMO
There's a tendency in many cases to believe gnat's-ass tight tolerances are best. Too tight ain't good- whether it's thread fitment, bolt-nose clearance, neck clearance, freebore diameter, bolt raceway clearance, etc. Especially not factoring in the environment the rifle will be used..."one size" does NOT fit all.

There's a reason you can bury an AK in a pile of sand, pull it out and it'll go bang.

 

[DaveTooley]

JMO
There's a tendency in many cases to believe gnat's-ass tight tolerances are best. Too tight ain't good- whether it's thread fitment, bolt-nose clearance, neck clearance, freebore diameter, bolt raceway clearance, etc. Especially not factoring in the environment the rifle will be used..."one size" does NOT fit all.

There's a reason you can bury an AK in a pile of sand, pull it out and it'll go bang.

The law of small numbers will catch up to you.

 

[Rich S]

Thanks for all the good information.

 

[Laurie]

I like to have between .0003 and .0005 actual clearance on the diameter. The thing is, bullet diameter does vary a few tenths.

I saw this in a couple of 6.5 Creedmoor rifles in the cartridge's early days. Total nominal throat clearance is only a half-thou for this design, and this apparently defeated some rifle manufacturers, at least initially.

David Emary (retired from Hornady) wrote a fairly long piece for Lyman's Long Range Precision Rifle Reloading Handbook on the cartridge's background and development and waxes lyrical on PAT minimisation.

"Limit and control as much as possible Principle Axis Tilt (PAT). ................. It is very simple, force the projectile to enter the rifling as straight and true as possible. In other words don't let it tilt when entering the rifling and being engraved.

............ The best way to design a throat is to provide approximately 0.0005" between bullet and throat."

And so on and so forth.

I soon found that the 139 gn Lapua Scenars I initially tried loading in my first factory Creedmoor, a newly introduced Savage 12 LRP in the chambering, wouldn't pass through the throat without a hard shove as they were an interference fit. Measuring the bullets came up with 0.2645", and the throat was apparently marginally under that. Most of my 6.5 match bullets exceeded 0.264 diameter and suffered this. Nothing that produced the condition would shoot reasonably. Eventually I eventually found a few overlooked boxes of 140gn Nosler Custom Competitions, a low-BC tangent design that I'd relegated to occasional short-distance use and they were the only bullets I owned that would slip through the throat without friction. Finally, the rifle grouped well, in fact even at 1,000.

I went off the tight PAT criterion, and for that matter, the cartridge as a result of what I consider a stupidly small clearance spec. As there are plenty of happy 6.5 Creedmoor factory rifle owners around, I can only assume manufacturers received feedback on this issue and increased clearances a bit.

 

[urbanrifleman]

My 223 ISSF barely cuts the groove in my barrels and it shoots wonderfully. Is crazy forgiving and shoots barrel after barrel. It is 5 ten thou over.

My 223 230 fb cuts away the entire bore (as most "standard" reamers do).

I can't say one is better than the other. I don't think anyone can.

Also, I was told by more than reamer maker that the tolerance is always over on fb diameter. Never under the print.

 

[Alex Wheeler]

So what happened to the "bore rider" fad?

I have never been a fan. Yes, I have tried it in some different things even BR. In efficient cartridges I didnt see a velocity gain, in the large stuff its worth a node. But it also cuts down on accuracy life. Your removing half the rifling. Some guys do use them and with success. But I dont.