The Voyeur's Guide to Chamber Throating
by Germán A. Salazar and John Lowther
 |
| PTG Throating Reamer with PTG Micro Adjustable Reamer Stop attached |
We're back in John Lowther's shop this month, looking over his shoulder as he uses a
PTG throating reamer to extend the throat length on the chamber he cut for me last year. You can read all about the chambering job by
clicking here. As with the chambering articles, the purpose of this piece isn't to teach a gunsmith how to do this; it's a primer for the customer that will help him understand the process he's paying for and perhaps to appreciate the gunsmith's work a bit more.
 |
| John Lowther putting barrel in the lathe |
When John chambered the barrel on my F-TR rifle, we were in a real time crunch because the Arizona Long-Range State Championship was a couple of weeks away and we didn't have time to get the reamer I wanted, nor did we have time to ship the rifle to
Clark Fay who normally does my barrels. Clark has the reamer that I prefer, which has a 0.114" freebore. Freebore is the throat length plus chamber to throat transition length. If you look at a chamber print, you'll see the two dimensions clearly.
That 0.114" freebore length allows me to seat any bullet from a 175 gr. Berger LRBT to a 190 gr. Sierra Match King with the base of the bullet shank above the case's neck-shoulder junction. I have a fairly firm belief that getting the shank below that point can deteriorate accuracy; and it certainly eats up powder space which I really need with the slow, bulky powders used with heavy bullets. More on that topic
here.
We can wrap ourselves into knots worrying about seemingly insignificant details and this may, or may not be one of those situations. With the original 0.050" freebore chamber that John cut in November, I won the Arizona Long-Range Championship, the Arizona Palma Championship, the Arizona Mid-Range Championship and was one point out of first place in the Berger SW Nationals. Obviously, it wasn't too terrible a chamber. However, almost all of the shooting in those matches was done with the Berger 175 gr. LRBT because it could be seated above the neck-shoulder junction. I shot some of the 1000 yard stages with the Berger 185 gr. LRBT despite it being below the junction, and had a best score of 199-11X at 1000 yards with it; that's one X short of the F-TR national record. Was there anything really wrong with a 0.050" freebore?
 |
| PTG Throating Reamer 0.5 degree x 0.3084" |
A throating reamer is simply a straight reamer of the diameter that is appropriate to the chamber in question and, most importantly, with a leade angle matching that of the original chamber. The chamber reamer used originally has a 0.5 degree leade angle, not the more common 1.5 degree angle. Accordingly, John ordered a 0.5 degree leade angle throating reamer with a 0.3084" diameter to match the chamber.
The front end of the reamer accepts a bushing, just like the chambering reamer; that is very closely matched to the barrel's bore diameter. We'll get back to that in a moment.
Normally when I'm in John's shop, my job is to take pictures, take notes and keep my hands and finger away from the sharp, moving parts. For some inexplicable reason, the thought of a bleeding, screaming lawyer searching for a lost finger in his shop doesn't fill John's heart with mirthsome glee...
This time, I actually had a task to perform: check the seating depth for the five bullet samples we determined to be relevant. Before the barrel was put into the lathe, I used my Stoney Point tool to check each bullet (two samples of each) and made a record of the seating depth for each.
The bullets we were checking are:
1. 175 gr. Berger LRBT
2. 185 gr. Berger LRBT
3. 180 gr. Sierra MK, short boat tail
4. 180 gr. Sierra MK, long boat tail
5. 190 gr. Sierra MK
We recorded the base to ogive length for each bullet when touching the lands, and would check and compare frequently during the throating process.

Next, John slipped the barrel through the headstock and indicated it in to center just as he does when chambering a barrel. John is a careful and methodical practitioner of the metal cutting arts and I appreciate the extra time and effort he takes to make sure that there is no margin for error. Even though this step isn't absolutely necessary, I am confident that it leads to a better finished product.

Once the barrel was centered up properly, John tried a few bushings to find the best fit for this barrel. This involves installing the bushing on a range rod - that's a very straight section of rod made for this purpose - and slipping it into the barrel. He's looking for a slip-fit that just barely touches the bore all around, allowing no slop and no binding. The bushing rotates on the range ro as it will on the reamer, but a close fit is what will keep the reamer perfectly centered in the bore and in alignment with the chamber. This seemingly simple step that relies on the gunsmith's feel for the tightness of the fit is fairly critical to a good job.
Next, the PTG micro adjustable reamer stop is installed on the throating reamer (see top picture) and cutting lube is applied to the reamer. The reamer is slipped into the bore and turned backwards until it makes contact with the rifling. The reason it is turned backwards is to avoid cutting while the initial depth is located. The micrometer brought into contact with the and of the barrel, the setting is noted the cutting can begin.
The micrometer is dialed back the desired amount to create clearance between it and the back of the barrel. That will allow the cutting edges of the flutes to advance the same distance into the rifling, thus advancing the throat. Our objective was to move the throat forward 0.050", so as a conservative start, John dialed 0.010" on the micrometer, inserted it, applied gentle pressure with his hand while holding the back end on center with the tailstock, and rotated the headstock by hand to perform the actual cut.

Once that first cut was done, I re-measured the seating depth for the various bullets and we were surprised to see that the throat actually moved forward close to 0.030". We also noticed that not all of the bullets indicated the same amount of movement; that made me glad I'd decided to take a large selection of bullets, not just one type.
John dialed 0.010" more onto the micrometer and went at it again. This time, he said that rotating the headstock produced a distinct cutting feeling each of the four lands. That was something he didn't feel on the initial cut which he now described as feeling more like cutting charcoal. Another check of seating depths after this cut showed that it was actually 0.010". I should mention that the barrel had 1150 rounds fired through it at this point and looked great in the borescope. It definitely wasn't a worn out barrel, yet that first cut was not what we expected.
John then took two more cuts, each one at 0.005" and the bullets generally advanced that amount. At that point, almost all of the bullets showed an advance in their seating depth to the rifling of 0.050" total, and the 190 gr. Sierra finally had the shank past the neck-shoulder junction. We called it a day and retreated to the local diner for breakfast. The whole job took about two hours not including disassembling the rifle, which I did the previous evening.

I went home, reassembled the rifle, and loaded a set of ammunition for the next day's match. I used the Sierra 180 MK, moly-coated, pointed and jammed 0.015" into the rifling with IMR 4064 powder.
Sunday morning dawned warm but not too windy; we had a 500 yard match at the Phoenix Rod & Gun Club. I was pleasantly surprised when the first shot out of the barrel hit the 10 ring after all of the disassembly, throating and reassembly. It's a well built rifle, so perhaps I should have expected that, but I didn't.
The first string went well with a 200-13X and I was certainly pleased with that. I was a little nervous and hesitant on the trigger during the second string, resulting in a 200-8X; still good, but I knew my hesitation allowed the wind get me into the 10 ring too often. The third string brought a more decisive trigger break and resulted in a 200-15X under virtually the same conditions as the previous two strings. The total score, 600-36X, beat my previous high F-TR score by three points and was my 24th score of 600 points in a mid-range match. However, all the previous 600's were in the sling on the standard target, so this was really a noteworthy match for me - my first F-TR 600!
You would be safe to say that I'm very happy with the outcome. The rifle and load load shot very well, and held X-ring elevation for at least 50 of the 60 record shots. All 66 shots fired (six sighters and sixty for score) were in the 10 and X rings. I can't think of a better proof of the quality of the work done, and perhaps we did see that slight accuracy edge gained by getting the shank of the bullet away from the neck-shoulder junction. At a minimum, we did no harm.
 |
| Berger 185 gr. LRBT before (above) and after (below) throating. Seated to equal 0.020" jam in rifling. |
 |
| Sierra 190 gr. Match King before (above) and after (below) throating. Seated to equal 0.020" jam in rifling. |