The measurement from a 1.520 30BR case to the rifling of my barrel is .047. I have read this is suppose to be around .020”. I think this is called the freebore? These are new brass casings so I assume these will stretch after they are fired. What is the maximum this gap should be? Is it more important this distance or the bullet seating depth? The measurement of the chamber is 1.567”. The barrel is a Hart 21” 30BR.
30BR FreeBore
- Thread starter MkGram
- Start date
'Free bore' is the distance from the end of the chamber neck to the start of the rifling.
If I'm reading this correctly...you're saying you have .047" from the end of the case neck to the end of the neck area of the chamber? In other words...a .047 'gap' at the end of the case neck when it's in the chamber? -Al
If I'm reading this correctly...you're saying you have .047" from the end of the case neck to the end of the neck area of the chamber? In other words...a .047 'gap' at the end of the case neck when it's in the chamber? -Al
Yes, that is correct.
mwezell
Mike Ezell
Those numbers are plausible for some reamers. Can you explain how you got the 1.567 dimension from your chamber? That's longer than is typical but not impossible.
Here's an excellent thread, by Randy himself
https://benchrest.com/showthread.php?97838-The-so-called-Robinet-30-BR-reamer-exposed!
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I have a tool made from a 30BR case which has a blunt rod that is a might larger diameter than .308. It slides into the case when the bolt is closed. It is then extracted from the chamber and measured. It is a very precise tool so I know it is 100% accurate.
mwezell
Mike Ezell
Just checking. It's best if it measures very near neck diameter but it sounds like it's plenty sufficient. Here's one by sinclair but yours sounds good too. I usually make my own from brass or aluminum, just under neck diameter and left basically sharp. As you can see from the thread I linked above, it likely explains the issue you have. It also can be lived with...or even preferred by some.
https://www.brownells.com/reloading/measuring-tools/case-length-gauges/sinclair-chamber-length-gage/
You didn't say exactly what diameter the end of chamber length gauge is....that's important to know before going any further with advice.
It should be close to the neck diameter of the chamber. If it's just a little over bullet diameter, it may be protruding past the end of the chamber neck and being stopped by the tapered section of the rifling, rather than the actual end of the neck area.
For the .330 neck chambers, I make them .324 diameter. I just used this one a couple weeks ago to verify the length of an unknown 30BR chamber.
Check that diameter and let us know, ok? -Al
For anyone interested...here's another reason that chamber length plug gauges leading end should be significantly larger than the bullet diameter. On this reamer print, the circled area is the tapered area ahead of the case neck that leads to the beginning of the leade/free bore section. As you can see, this tapered area is .0117 long (1.520-1.5317= .0117). I have some prints here showing a .025 tapered section.
With tolerance stacking and normal reamer tolerances (usually .005 on length), it's entirely possible for a small gauge plug to be up into the tapered area and not at the end of the neck area of the chamber (the .332 dimension in the circled area).
If you've got a $69.95 Teslong bore scope, you can close the bolt on a case in the chamber and come in from the front of the barrel and actually visualize the gap between the end of the case neck and the end of the chamber neck. If you've got a shorter bore scope like a Hawkeye, just drill the primer pocket open to let the bore scope in and look at it that way.
Not sure if the O.P. is still monitoring this or has found the issue. Hopefully he is because not getting to the root of what's going on is the first step down a potentially nasty trail of tears.
There's a fair number of 'long necked' 30BR reamers out there that are meant to be fire formed in a 6BR barrel with the neck opened up to .330-.332. This leaves the case neck around 1.545-ish long....which may be exactly what the O.P. has. Many of these reamers were sold as '30BR Robinett' reamers...which they most certainly are not. In this scenario, unless the person doing the chambering and the end user know exactly how to come up with the long necked cases, they are going to end up with from .025-.045-ish of chamber neck diameter free bore. This will guarantee extremely poor accuracy and have the poor owner chasing his tail to find the cause.
Good shootin' -Al
With tolerance stacking and normal reamer tolerances (usually .005 on length), it's entirely possible for a small gauge plug to be up into the tapered area and not at the end of the neck area of the chamber (the .332 dimension in the circled area).
If you've got a $69.95 Teslong bore scope, you can close the bolt on a case in the chamber and come in from the front of the barrel and actually visualize the gap between the end of the case neck and the end of the chamber neck. If you've got a shorter bore scope like a Hawkeye, just drill the primer pocket open to let the bore scope in and look at it that way.
Not sure if the O.P. is still monitoring this or has found the issue. Hopefully he is because not getting to the root of what's going on is the first step down a potentially nasty trail of tears.
There's a fair number of 'long necked' 30BR reamers out there that are meant to be fire formed in a 6BR barrel with the neck opened up to .330-.332. This leaves the case neck around 1.545-ish long....which may be exactly what the O.P. has. Many of these reamers were sold as '30BR Robinett' reamers...which they most certainly are not. In this scenario, unless the person doing the chambering and the end user know exactly how to come up with the long necked cases, they are going to end up with from .025-.045-ish of chamber neck diameter free bore. This will guarantee extremely poor accuracy and have the poor owner chasing his tail to find the cause.
Good shootin' -Al
I never said you want the end of the neck there. But you do need to find where the end of the chamber is.
end of neck dia and end of chamber are two diff numbers. and only the neck number is truely useful.
are you saying end of chamber is begining of throat ??
mwezell
Mike Ezell
Pretty certain you and Al are talking the same thing. That's why he mentioned earlier that the plug gauge needs to be very close to chamber neck diameter, so that it hits as close as possible to the end of the neck/chamber where it's still full diameter.
I guess you could consider that 45° bevel as part of the throat because it does affect the amount of freebore but it's not technically part of the throat. I still haven't done it but I plan to spec a reamer that uses a 15° angle there vs the 45° that is most common. It'll make that area 3x as long so reduces the actual freebore length accordingly. My thoughts on it are strictly extending bbl life. I don't think accuracy should be affected either way but that smoother transition hopefully creates a much better heat sink to slow erosion and fire cracking at that point. Another one of those projects that I intend to do but haven't made time for yet.
consider doing the opposite. leave the 45 but shorten the gap from neck to gap to 5-7 thou. flame never sees the 45..just blows on by...maybe
mwezell
Mike Ezell
We're on completely opposite ends of the spectrum on this, logically.
Virtually every bbl I've ever seen, that area is precisely where erosion and fire cracking begin. That's why I like testing stuff though...so I know for sure or to my own satisfaction, the result of changes.
Another example is smith and Wesson. When they started making those huge honkin 500 s&w revolvers they suffered from flame cutting of the top strap of the frame. The fix was simple enough. They polished the inside of the top strap so the flame did smoothly flow over the smooth surface.
My goal is essentially the same, to smooth that transition area up and reduce steep angles and sharp edges that are prone to the propagation of bore erosion.
We'll see how it goes in time.
Testing different approaches and modifications is indeed a valuable way to gather firsthand experience and determine the results that work best for your specific setup and goals. Your intention to smooth the transition area and reduce erosion-prone features is a logical approach.
subway surfers
subway surfers