Free bore diameter?

[NYDAN]

What is the proper freebore diameter for a rifle?

Utilizing a RCBS Precision Mic I loaded some cartridges last night with bullets with a secant ogive. I had previously established what I thought was the RCBS Precision Mic measurement for contacting the lands using Nosler bullets with the tangent ogive.

I was loaded several OAL (based on ogive dimensions). All should have cleared the lands by at least .018" and up to .035".

To my great surprise, the cartridges would not chamber. When I inspected the cartridges after extracting them from the rifle I carefully inspected them and saw scuff marks all around the major diameter (0.277") of bullet just behind the ogive.

I smoke blackened a bullet and checked the OAL (ogive reading) at which the bullet contacted the lands. It was obvious that my contact point was the beginning of the free bore diameter, not the lands of the rifling.

It appears to me that with the secant ogive bullets, the bullet wedges in the free bore diameter before it contacts the lands.

I need to double check what the bullets with the tangent ogives are contacting first.

Shouldn't the free bore diameter be greater than the diameter of the bullet. Shouldn't there be some small clearance? The bullet shouldn't jam in the free bore area should it?

Dan

 

[skidmark]

I always thought freebore was the same as bullet diameter.
Bullet should go through. ?

 

[SJB358]

I am watching this closely, but I always believed freebore was just a hair larger than the caliber being used to lessen the initial pressure created and give the bullet some travel before hitting the rifling.

 

[Dwh7271]

freebore was just a hair larger than the caliber being used

If it's not that then I sure have messed up on some barrel builds. :grin:

 

[Ridgerunner665]

SAAMI drawings indicate .0005"-.005" over bullet diameter....it varies greatly.

 

[Ridgerunner665]

Carbon ring?

 

[nvbroncrider]

SAAMI drawings indicate .0005"-.002" over bullet diameter....it varies some.

Example....7mm RM is .0005"

308 Win is .002"

Right which is why the lands is generally your first point of contact with the bullet.

 

[longrangehunter]

I am watching this closely, but I always believed freebore was just a hair larger than the caliber being used to lessen the initial pressure created and give the bullet some travel before hitting the rifling.

+1

A hair larger, .0005" and quoted by the length, ie: .235" free bore or whatever was determined for a given bullet/case design.

The 7mm-08 is only .119" free bore and tapers at 3 degrees for .0716" from the end of the free bore.

The area cut at the end of the chamber for the case is the area giving you the grief I assume? Is this the first time you've noticed this? What about other bullets?

 

[NYDAN]

Carbon ring?

RR, I don't think so. I have only shot 19 rounds through it since I cleaned it. I plan on cleaning the rifle tonight and that should remove any carbon. Then I will check everything again.

I am wondering if there is some kind of burr.

Dan

 

[NYDAN]

The area cut at the end of the chamber for the case is the area giving you the grief I assume? Is this the first time you've noticed this? What about other bullets?

Yes, the area that the drawings call the freebore or leade. Unless there is another step between the neck area of the chamber and the rifling.

Interestingly, I have had a lot of trouble trying to establish the OAL (or ogive measurement) where the bullets with the tangent ogive contact the lands. The bullet usually gets stuck in the barrel and I have to poke it out from the muzzle with a cleaning rod. Also, the readings have varied by quite a bit. Now, I am wondering if the gradually taper of the tangent ogive allowed the bullet to wedge into the the freebore.

I am going to clean the rifle tonight and smoke blacken some bullets with the tangent ogive and check to see if they are contacting the lands or something else.

Dan

 

[longrangehunter]

Dan,

Since it's a new gun the chambers throat could have been cut wrong? I've seen lots of things that somehow seems to go wrong? Really. Sometime they use two reamer to cut the neck size and/or throat area for free bore. Who ever did the work I'd give them a call. There are two types of throats, funnel and parallel. Most are funnel, but there are a few that use parallel method. Either way it sounds from what your seeing trying to fund an aol measurement variance tells me something is up. Look your once fired brass over real well, anything out of place the brass will flow into it when fired, i.e.; tooling marks, check neck size, and case dimensions. The throat maybe just cut wrong/not enough/ something isn't right from the sound of it?

 

[NYDAN]

I think I know what is going on but it is a long explanation. I will try to write it all up tomorrow when I am more rested.

I will need to find a gunsmith with a bore scope to verify what I think.

Dan

 

[truck driver]

Dan, I'm just curious if you didn't get some over sized bullets from Sierra and that might be your problem since the Noslers chamber and the Sierras won't.

 

[NYDAN]

I am sorry I haven't been back on the forum for a few days. Work got crazy and I have been exhausted every evening since my last post. I was literally falling asleep as soon as I ate dinner.

As I said above, I think I have an explanation. However, I managed to get an appointment with a gun smith tomorrow morning and he is going to cast the chamber and make some measurements. Then we won't have to guess as to the explanation.

I will post the results of examining the chamber cast as soon as I can.

Rodger, the new bullets are the 140 gr. Hornady BTSP Interlock with the secant ogive. They measure .277", the same as the nosler bullets. I think the explanation has to do with the shape of the ogives. The tangent ogives of the Nosler bullets have a much gradual decrease in diameter from .277" to .272" diameter (the diameter of the ogive gauge in the RCBS Precision Mic). I am thinking the gradual decrease in diameter allows the bullet to contact the lands before the major bullet diameter contacts the groove diameter of the barrel. The the secant ogive of the Hornady bullet has a much faster decrease in diameter from .277" to .272". That allows the major bullet diameter to contact the groove diameter of the barrel before the .272" diameter contacts the lands.

The question is why is the major diameter .277" of the bullet wedging into the groove diameter of the barrel before the bullet contacts the lands. Is the free bore diameter the same as the groove diameter? Is there a burr between the leade and groove diameters? Hopefully the chamber cast tomorrow will provide some insight.

 

[NYDAN]

I am back from a great visit with the gunsmith. He was very personable and very knowledgeable. He knew a lot about reloading and accuracy shooting. I saw newspaper clippings on the wall naming him as the "Top Shooter" for some shooting events. He said that he had built some target rifles for some clients.

I showed him what I was concerned about and he agreed that it didn't look normal to him either. So, he made a cast of the chamber. What he found was that:

1. The leade was relatively short.
2. The diameter was relatively tight.

He was measuring the leade as the same diameter as the Hornady bullet. Which is why I was seeing scuff marks on the Hornady bullets. He thought the Nosler bullets measured a couple of ten thousands of an inch less. The problem wasn't obvious wtih the slightly smaller diameter of the Nosler bullets and the long tangent ogive. But the slightly larger diameter of the Hornady bullets and the sharp drop in diameter of the secant ogive brought the issue to light.

After some discussion of options such as trying to incease the diameter of the leade, we concurred that the best option was leave it alone for now. He stated that accuracy is generally considered better if the throat has minimal clearance.

I think it might be best to try to match the seating depth of the bullets so that the major diameter of .277" of the bullet was just behind the .277" diameter in the leade. Trying to utilize the contact point of the lands with the bullet doesn't seem relative if the .277" diameter of the bullet is contacting the .277" diameter of the leade first.

It appears that concentricity will be very critical since any misalignment would not allow the cartridge to chamber without the bullet binding in the throat. A concentricity gauge and a neck turner might be helpful since the tolerances are so tight.