How would you... ?

[kendog]

KIA!!??

No Soup... Two Years!

95% of the JGS prints I have seen have the throat datum info on them. Usually it is .002" under groove size, to the length specified on the print for that specific reamer. It's good to know, because you can seat ANY bullet pretty much where you want it in that throat, and know where it is in relation to the lands. I supply this data with all barrel jobs so the shooter can use it when developing loads.

 

[memilanuk]

The print I have in hand at the moment is from PT&G, for the new 2013 US FTR team reamer. Basically a '95 Palma with .170 freebore. The only diameter it lists forward of the .3085 for the lead is .293 , which I'm not sure what exactly thats supposed to correlate to.

 

[kendog]

Welllll....

I am just as confused as you are. .170" from where?? What is the throat angle of your reamer? 95 Palma is 1deg,45 Min, .050" of leade (or freebore), and the throat datum is 2.1245. The tenth is a little overkill for the shooter as I am sure it changes with a very few shots.

Call PTG and ask for the info.

 

[memilanuk]

The 0.170" 'lead' is shown as being from the end of the chamber mouth to the beginning of the throat (where the 1 degree 30 minute taper to the rifling starts).

 

[Gundoktr]

Ha it occurred to anyone here that those dimensions are for the reamer manufacturer; for him to determine how to set up his CNC grinder?

SOmeone orders a reamer with some non-stndard dimensions, like a .170 freebore, and he punches the data into the program and "Voila" instant spec reamer,.

 

[kendog]

The Obermeyer chamber has a tapered neck, .346" to .344", and yes, most of the dimensions on the prints are for the tool maker.
The 3085 dimension mentioned earlier is not the neck.

 

[memilanuk]

Not sure where this came from... don't think I said the *neck* was .3085. The 'lead' aka freebore is listed as such, though.

 

[kendog]

Gundoktr did in post #14. No big. There are a bunch of ways to skin this cat. I thought your original question was to try and understand what/where the dimensions and terminology meant. For me, the throat datum info is the foundation for all bullet seating depths, regardless of lead and throat differences.
You could throat your Obemeyer chamber to the 2013 FTR reamer spec, but your neck will still be tapered and a little larger than the cylindrical Palma neck, assuming that is the only difference.
PM me your email and I can send you a print that clearly shows what is considered the leade and throat.

 

[memilanuk]

Ah. I didn't read back that far

I've got reamer prints for various chambers from both PT&G and JGS... they all show sections labeled 'lead' and 'throat' pretty clearly. They all have a number (diameter) listed out past the end of the chamber or throat - ranging from .293 to .295 to .297. None of them list .306 any where.

Where I got thoroughly confused was (usually) the dimension marked as 'lead' is too short and the length of the region designated as the 'throat' (to the given .29x diameter) would be way too long - somewhere over .200" for an Obermeyer chamber.

I'm trying to make sense of what the term 'throat' means when tossed about in casual conversation between shooters, as in "my gunsmith cut the chamber with 0.nnn" throat for brand X bullet", or "...used a '95 Palma reamer throated an extra 0.120" long." Neither the dimension 'lead' nor the dimension 'throat' on any print I've seen yet matches up with those discussions. Short of hunting down *their* gunsmiths and digging out of *them* what exactly they meant... dunno as I'll ever know.

 

[kendog]

The throat datum on most JGS prints is at the bottom left corner of the page, usually under Comments. Typically, my 308 Obermeyer print says "Comment: Throat datum .306 X 2.1564". This is the point where the bullet ogive at .306 diameter will just touch the lands on a new throat.

The way I understand it, the leade, or freebore, is from the end of the neck to the origin of the rifling, and the throat is the actual angled area of the rifling that is cut away. I think this is also referred to a ball seat.

I agree, there is a lot of ambiguity with reference to these points.

It makes me thirsty!