Gunsmithing Normal freebore diameter/clearance?

Keith Johns

Keith Johns

Phoenix Custom Rifles
Commercial Supporter
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Minuteman
Jan 7, 2009
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Arizona, USA
www.phoenixcustomrifles.com
#1
Whats the average or normal freebore diameter over a given caliber bullet?

Use .308win for an example. Would freebore be .0005" over in diameter? .001" over? Whats the norm?


Dave Kiff, if you read this, what how much oversized do you make your 308 Tac Match reamers freebore?

Again, just talkin diameter here, not freebore length.

Im running into some factory 175gr loaded ammo hitting freebore due to the bullet diameter and the freebore diameter being the same size. As you can imagine the rounds wont even begin to chamber, and the bullet gets stuck in freebore/lands if you force it closed.


Is it possible my reamer was ground a little too far? Or worn??

Thanks a ton.
 
#4
Re: Normal freebore diameter/clearance?

Free bore and loads not chambering has been the hardest thing for me to troubleshoot. It's also very hard to see where in the chamber thing are interfering. I suspect it's at dimension "G" and "H" and the distance from the bolt face to where "H" starts.

eqqdm0.jpg
 
#5
Re: Normal freebore diameter/clearance?

There is no such thing as normal. It all depends on the application and if you are needing to account for variance in bullet diameter or not, based on the QC specs for the bullet and its intended use.

Generally, a true match chamber than can count on being fed quality match bullets is 3 to 5 tenths over bullet diameter. If you need to account for the variance of ball/military ammo, then the number gets bumped quite a bit.

Have someone send you the prints for an M852 chamber and a Warner 95 Palma chamber, then compare the two prints. The M852 chamber is made specifically for the M-14 national match rifle that will be fed a diet of M852 match ammo (with 168 SMKs) yet still can accommodate M80 Ball for practice and 200 yard match use when match ammo is scarce.

Back to your comment about the bullet dragging on the freebore, when you have 3 to 5 tenths, it takes very little runout in the round for the bullet to drag a tad on one side of the freebore. With 3 tenths clearance in my Palma chamnber, it is very common for me to drag the bullet a bit on ammo I consider more than adequate runout-wise for 1000 yard match competition.
 
#6
Re: Normal freebore diameter/clearance?

Well, I have a rifle that wont chamber factory BlkHills or FGM 175s loaded with SMKs. Hornady 175s will chamber, but they still contact freebore. Just not as tight as the SMKs.
Its a 308 Tac Match reamer being used... so pretty standard.

The bullets are very tight in freebore. One got pulled from the case when the case from extracted. Theyre hitting all the way around evenly, wayy before they should be. .100" before lands Id guess. Right after the case mouth.


I need a solution.

I guess I'll find out what PTGs Tac Match reamers are ground to and get one ground a little larger. Like .0005" larger.


Why would my reamers freebore dia be so tight? Is it likely it got ground too small from the factory? Or do is it more likely that it's worn down from use? This is the first time Ive had this problem with it, BUT, we have noticed before that freebore seemed extremely tight using this reamer because of signs on ammo. So I have seen signs of this before.
How many uses should you get from a HSS reamer before this dimension may decrease? The reamer has been used, but used lightly.
 
#7
Re: Normal freebore diameter/clearance?

What does the reamer measure? Measure the specific reamer you used for that chamber and see how it compares to the blueprint.

A separate throating reamer, or running a chamber reamer of known dimension into the chamber by hand (using cutting oil smears or lack thereof at the shoulder) will solve your problem in little time one way or another.

Measure the reamer would be the first step. It could be undersized from wear or when cut originally. You would not be the first person that experienced either or both situations.

Asking someone else about your own reamers is rather counter intuitive to me - you should have the blueprints on hand and the equipment on hand to measure them when they come from the mfr, long before you approach a barrel with them. Cerrosafe is also an important thing to have on hand when diagnosing chamber issues.

http://www.brownells.com/.aspx/pid=384/Product/CERROSAFE-reg-CHAMBER-CASTING-ALLOY

 
#9
Re: Normal freebore diameter/clearance?

Ream the cutoff stub and it can be used as a gauge to tell you mucho. I recall someone have a lot of ammo that had oversized bullets recent too.
 
#12
Re: Normal freebore diameter/clearance?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: hero's machine</div><div class="ubbcode-body">Ream the cutoff stub and it can be used as a gauge to tell you mucho. I recall someone have a lot of ammo that had oversized bullets recent too. </div></div>

From past experience, meaning it didn't work out as well as planned, just reaming a short section of barrel will give you a false reading. It will measure a few tenths over size. The reason is the reamer is not fully supported.
 
#15
Re: Normal freebore diameter/clearance?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SSC</div><div class="ubbcode-body">PTG Tac Match is .3085 </div></div>
The JGS is .3083.
Both are still bigger than the bullet so where is the interference.
 
#16
Re: Normal freebore diameter/clearance?

Here is a pic from a recent fix of a supposed good chamber done by a top builder in their respective field. This is an interim cut - note the oil still present for about .200" on the body before the body/shoulder junction and the oil intact on the shoulder. This is a JGS Wylde reamer BTW.

Before someone throws up all over using a Tee handle and doing this by hand, there are some top gunsmiths out there that do the entire chamber with the reamer held in their hand. It is not difficult if you are careful to to touch a chamber up this way - just clean the reamer off with air and brake cleaner, then re-oil for every pass. You inspect the reamer with magnification after you pull it out and before cleaning to see where you are depth-wise.

Bottom line is it turned the rifle into something usable from unusable and produced accuracy exceeding the demands of its intended use with standard loads for the chambering, something it could not do prior.

handream1.jpg
 
#17
Re: Normal freebore diameter/clearance?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: waste_knot</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: SSC</div><div class="ubbcode-body">PTG Tac Match is .3085 </div></div>
The JGS is .3083.
Both are still bigger than the bullet so where is the interference.
</div></div>


Just because my reamer is s<span style="font-weight: bold">upposed</span> to be at .3085, doesnt mean it is.

I have my doubts.


If my reamers freebore was bigger than my .308 bullets, how would they be hitting hard all the way around and getting stuck? From what I've seen, I think my reamers freebore is like .3081 or less somehow. A .308 bullet should go into a hole greater than .308 without getting stuck.... right?
 
#18
Re: Normal freebore diameter/clearance?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: _9H_Cracka</div><div class="ubbcode-body">Here is a pic from a recent fix of a supposed good chamber done by a top builder in their respective field. This is an interim cut - note the oil still present for about .200" on the body before the body/shoulder junction and the oil intact on the shoulder. This is a JGS Wylde reamer BTW.

Before someone throws up all over using a Tee handle and doing this by hand, there are some top gunsmiths out there that do the entire chamber with the reamer held in their hand. It is not difficult if you are careful to to touch a chamber up this way - just clean the reamer off with air and brake cleaner, then re-oil for every pass. You inspect the reamer with magnification after you pull it out and before cleaning to see where you are depth-wise.

Bottom line is it turned the rifle into something usable from unusable and produced accuracy exceeding the demands of its intended use with standard loads for the chambering, something it could not do prior.

handream1.jpg
</div></div>


lol maybe its too early for my brain to work, but whaaat? Are you off in left field or somethin? haha

((Off Topic)BTW 9H I love your signature. None is my favorite religion.)
 
#19
Re: Normal freebore diameter/clearance?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Keith at PCR</div><div class="ubbcode-body"> A .308 bullet should go into a hole greater than .308 without getting stuck.... right? </div></div>

I didn't see if this was a brand new chamber or something that has developed as you were shooting it.

Can you tell us what point the chamber is at?

New, unfired
A couple rounds ( less than 20)
21-200
200+

I'm assuming it's one of the first 3 and not the last, and if I missed it, my apologies.

If it's in the last 2, scrub the thing for a carbon build-up and pour some cero-safe or molten sulfur in there.
 
#20
Re: Normal freebore diameter/clearance?

What do you mean "off in left field" - can't you see where the reamer has made contact with the existing throat and took out metal? (neck and body too). The subsequent insertions took out more metal in all 3.
 
#21
Re: Normal freebore diameter/clearance?

Gotcha. When I get the new reamer thats what Ima do... run it in on our T handle to cut out the problem area. I couldn't think earlier and wasn't sure what you were sayin'.
I think my damage has brain sometimes
smile.gif




I spoke with Dave today. I bought a new reamer with a little longer freebore, and it should have the same .3085 freebore diameter. So, whether my reamers freebore is too small in diameter from being worn down, or if dist to lands isnt long enough, this new reamer should tend to both concerns.

Dave was nice enough to rush the reamer along and I should be able to take care of my customers rifle very quickly.



John, sorry I had to put ya through this in the first place man! We'll make it up to ya!
 
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