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Measuring a 7-30 Waters throat

Posted: Fri Mar 08, 2013 1:37 pm
by mtngun
I may have posted this info before but if so, I couldn't find it. :lol: In any event, I wanted to make a couple of experimental bullets for my 7-30 Waters TC barrel so that's a good excuse to talk about how to measure a rifle throat.

The 2 basic methods are cerrosafe or an upset slug. I used to rely on cerrosafe because that's what the experts recommended, but after getting cerrosafe stuck in the chamber a couple of times, I decided I liked the upset slug method better. :lol:

You'll need a dummy case (no live primer) and a pure lead bullet. If you don't have a bullet mold in the right caliber, you can use a smaller caliber -- it doesn't matter if the bullet is too small because you're going to upset it to fit your throat.

I made a couple of pure lead bullets in a 7mm mold. The bullets were cold and wrinkled but that doesn't matter because I'm not going to shoot them. I filled the dummy case with rice up to the base of the neck so that the bullet could not fall inside the case. On other occasions I've poured lead into the dummy case up to the base of the neck to accomplish the same purpose.

Then I chamber the round with the pure lead bullet just barely started in the case. A little oil on the bullet may assist extraction later on. Then I inserted a brass rod through the muzzle until it came to rest against the bullet, and gave the rod a few smacks with a hammer to upset the bullet.

Dummy case with pure lead bullet ready to chamber.

If I had been using a too-small bullet, I would have chambered the empty case, dropped the too-small bullet down the muzzle until it came to rest against the case, then used the brass rod to upset the bullet.

Even if you do everything right, sometimes the upset slug will stick in the chamber and have to be pounded out. Sometimes the slug gets damaged trying to get it out. :? If that happens, just do it over until you get a slug that's in decent shape.

An upset slug (top) and a cerrosafe cast (bottom) of the same 7-30 chamber. You can't tell from the crappy photo, but the upset slug was actually quite decent and I had no problem getting consistent measurements from it, except on the bore which I'll discuss later.

To measure the bore, I like to use an expanding ball gage, inserted at the muzzle. I think I paid about $10 for this set of Asian gages some years back. It does take some "feel" to use the gages properly, but once mastered, they work well enough. However, the bore tends to erode just ahead of the throat, so it's likely the bore will be larger at the throat than at the muzzle. I decided not to worry about that.

Some people measure the bore at the throat by measuring the chamber casting or chamber slug with a blade micrometer. I tried doing that but it wasn't working because of the twist and the small caliber -- it's too difficult to fit the blades in the twisting rifling impressions. Also, the pure lead is easily dented by the micrometer. You can get a reading, but is it a reliable reading ? I did not trust the blade mic readings and decided to ignore them.

Some people like to use pin gages to measure the bore but in my opinion they are not very accurate. It takes significant clearance to insert a pin gage without force and without damaging the barrel and that clearance throws off your measurement. If a pin gage can slide into the barrel easily, it's prolly at least 0.001" smaller than the bore and maybe 0.002" or 0.003" smaller. That's not very accurate.

Final results:

Bore diameter at muzzle = 0.2785"

Chamber neck diameter = 0.3085"

case neck thickness (WW 30-30 case) = 0.011"

Throat/leade diameter = 0.286" tapering down to 0.285"

Groove diameter near throat = 0.285" (it's prolly eroded a bit near the throat, I think the rest of the barrel is closer to 0.2845")

Distance case mouth to rifling = 0.14"

Loaded with a 0.286" bullet, the WW case neck OD should be around 0.308" for a snug fit in the 0.3085" chamber neck.

Re: Measuring a 7-30 Waters throat

Posted: Fri Mar 08, 2013 2:01 pm
by mtngun
I'm planning to try these 2 designs and go from there. It should be an interesting shootout between the Loverin vs. the bore rider. I'm trying to keep it apples-to-apples as much as possible so they will be sized the same diameter, have the same GtoB, same alloy, etc., etc..

Why 120 grains ? For the velocity. No doubt a heavier bullet would be more accurate but I want to push these as fast as the puny 7-30 will allow, sort of a torture test to see what goes wrong and what works.

The Loverin.

The bore rider.

Re: Measuring a 7-30 Waters throat

Posted: Tue Mar 12, 2013 7:16 pm
by mtngun
Since the last post I tweaked the designs slightly by changing the design alloy to WW. I'll actually be casting with a mystery alloy that is approximately WW & 2% tin but I want my weight to err on the light side not the heavy side.

Anyway, the 2-cavity aluminum mold cast surprisingly well, throwing good bullets about 90% of the time once it had found its groove. I'm not a big fan of WW & 2% Tin but it does have its place with small caliber bullets.

One problem was that I forgot that the design page specs check shanks fatter than I personally like them. The design spec is 0.251" and these measured 0.2505" - 0.2517". That would be perfect for my customers, who seem to believe that a check is too loose if it can be turned with a pipe wrench, but personally I prefer an easy fit. As it was the checks needed some assistance seating all the way.

The lubrisizer was used to seat the checks. I swear I have a check seating gadget around here somewhere, but I couldn't find it, so I used a piece of scrap metal.

The trick was to seat the check hard enough to seat it all the way and square, but not so hard as to "bump" up the nose of the bullet. Sometimes I was successful and other times the nose was bumped up a thousandth or more. Before loading I'll have to measure every single bore riding nose and weed out any that are too fat -- what a pain in the ass ! :cry:

The bore riding nose dropped out at 0.2775" - 0.2780", pretty much where I wanted it. Being as this will be used in a single shot I wanted the bore rider to be an easy fit, with little or no engraving.

A homemade push-thru die was used to size the bullets to 0.2855".

The die pushes the bullets completely through rather than relying on the next bullet to finish the job.

Then the sized & checked bullets get a trip into the oven at about 435 degrees F for an hour.

Then the hot bullets get doused with water.

That's as far as I've got. A lot of my reloading and shooting stuff hasn't even been unpacked yet so this project will take a while.

Re: Measuring a 7-30 Waters throat

Posted: Sat Mar 23, 2013 4:17 pm
by mtngun
Bullets were lubed with Rooster HVR. The lube had to be applied by hand because my Lyman lubrisizer die would only fill the bottom groove.

It turned out that the Lyman seating die would mangle the nose of the bullet. I've loaded many cast bullets with these dies and never had a problem before. Maybe this being a compressed load made the seating punch work a little harder ? In any event, I solved the problem by using an RCBS 30-06 seating die.

Re: Measuring a 7-30 Waters throat

Posted: Sun Apr 14, 2013 3:04 pm
by mtngun
Finally had time to do some shooting ! :D

Unfortunately, my Pressure Trace refused to communicate with the USB port, so I skipped the Pressure Trace today until that can be debugged.

Also, the TC's trigger was misbehaving, releasing the sear as soon as you started to pull the hammer back. Like the Pressure Trace, the TC had been working fine last time I used it ..... 7 or 8 years ago :oops: ..... but developed gremlins while in storage. So I only shot a few rounds.

The scope wasn't sighted in for these loads, so I made scope adjustments between 3 shot strings, and this Leopold scope has been known to take several shots to "settle in" after adjusting, so take the groups with a grain of salt.

Load Data:
-- WLRM primers
-- WW 760 powder
-- WW 30-30 cases
-- heat treated WW & Sn
-- Rooster HVR lube
-- bore riders loaded to 2.583"
-- loverins loaded to 2.450"
-- target at 100 yards
-- 2.5X Leopold scope
-- 3 shot strings

-- 42 grain 760, bore rider = 2540 fps. 2 shots in 0.85", the 3rd shot missed the target (scope not sighted in for this load)
-- 43.5 grain 760 bore rider = 2638 fps. 3 shots in 2.0"

-- 42 grain 760 loverin = 2658 fps. 3 shots in 2.2"
-- 42.5 grain 760 loverin = 2693 fps. 3 shots in 3.5"

Picture of last 2 groups, remembering that the scope was adjusted between groups and may have been "settling in."

OK, so I didn't set any new benchrest records, but I was pretty happy with the results considering this project was a stab in the dark and I was dealing with equipment problems and 2700 fps is always a challenge with cast bullets.

The goal is to find a "decent" load for both of these bullets at about 2700 fps, using the same powder and primer for both bullets and only tweaking the grains and the COL to give the same velocity with both bullets. I'm not going to spend a lot of time tweaking in search of the perfect load, just something that looks decent, then shoot enough groups to determine which of the two bullet designs is more accurate.

Then I'll move on to a couple of experimental bullet designs, again loading them to 2700 fps with the same powder and primer, and again shoot enough groups to see which bullet design is more accurate. The goal is to see if one bullet design seems superior to the others.

Re: Measuring a 7-30 Waters throat

Posted: Sun Apr 21, 2013 12:12 pm
by mtngun
A few more shots fired today, mainly in an attempt to dial in my desired 2700 fps velocity, and also to get the Pressure Trace Computer working right. I've upgraded to version 4.5 of the Pressure Trace Software.

Unless otherwise noted all loads used WW30-30 cases, 120 gr. HTWW&Sn, Rooster HVR, and WLRM primers. COL was 2.59" on the bore rider and 2.45" on the loverin-ish bullet. This strain gage hasn't been used in 8 years so no guarantee if it is well calibrated, but the pressures do look believable.

Bore Rider with 44 gr. WW760, average 2664 fps, 40.6 ksi, 3 shots in 4.3"

Loverin-ish with 42.6 gr. WW760, average 2686 fps, 43.6 ksi, 3 shots in 3.1"

Then I tried the Loverin-ish with WW748 powder. 35 gr --> 2645 fps. 36 gr --> 2666 fps. 37 gr --> 2732 fps. 3 shots in 3.5"

Note the pressure and velocity really shot up at 37 grains ! :shock:

Then I tried the Bore Rider with Varget. 37 gr. --> 2614 fps for a single shot. The bullet hole was oval shape and the POI was 14" off normal. Note the secondary spike is much bigger than the WW ball powders -- there's a reason I prefer the WW powders for high velocity cast.

So what did I learn today. :?: I learned that 2700 fps is not going to happen at safe pressures with these 120 gr. bullets, so I'll dial my velocity back to 2650 fps. I learned that my initial choice of WW760 powder was a decent choice. WW748 also appears to be acceptable, but it is touchy when you lean on it.

I had intended to experiment with seating depth, but that's not going to be feasible with WW760 because the loads are heavily compressed.

The next step will be to shoot groups with both bullets at 2650 fps, to see if one design is more accurate than the other. Then on to experimental bullet designs.

Re: Measuring a 7-30 Waters throat

Posted: Sun Apr 21, 2013 12:51 pm
by mtngun
For the 2650 fps group shootout, I'll try 42 gr. WW760 for the Loverin, and 43.7 gr. for the Bore Rider.

Re: Measuring a 7-30 Waters throat

Posted: Wed May 01, 2013 3:28 pm
by mtngun

I started with a clean bore. One fouling shot was fired. Then one 3-shot group with the loverin, at 100 yards. Then one 3-shot group with the bore rider. Then another group with the loverin, then another group with the bore rider, and so on, until 5 groups had been fired with each bullet. The barrel was not cleaned at any time after the shooting started. Accuracy results are shown in the graph.

The Loverin averaged 3.55" and the Bore Rider averaged 3.08". An online t-test said the difference is not statistically significant.

The good news is that both bullets shot groups, not patterns, and group size did not open up appreciably after 30 shots, suggesting that fouling was not a huge problem.

Velocity results shown in the next graph. Loads had been previously selected to produce 2650 fps. The loverin actually averaged 2667 fps and the bore rider 2659 fps, measured 15 feet from the muzzle.

Shot #25 recorded 3392 fps, obviously a bad reading, so I deleted it. Shots #26 and #27 were also suspiciously high. Tthe light had changed about that time and it seemed to be giving the chronograph a hard time.

If the barrel had been fouling badly, I would expect velocities to gradually decline as the 30 shot string progressed, but instead velocities were pretty consistent other than the 3 suspicious readings.

Here's the pressure trace for the first two groups. First 3 shots are loverin, next 3 are bore rider. Nothing out of the ordinary here.

Here's the trace for the last 2 groups. First 3 shots are loverin, next 3 are bore rider.

I would have bet that the loverin would beat the bore rider at this velocity, but it was actually a toss-up.

Fouling does not seem to be an issue and at least the accuracy and velocity are consistent. This gives me a reliable standard that I can compare to other bullet designs (or bullet lubes, alloys, etc). So the next step is to try some experimental bullet designs, loaded to the same velocity with the same ingredients, and see if I can find a design that is superior.

Deets for today's loads. 120 gr. HTWW, Rooster HVR lube, WLRM primers, WW760 powder. 42 gr. in the loverin, 43.7 gr. in the bore rider. 2.45" COL loverin, 2.59" COL bore rider.

Re: Measuring a 7-30 Waters throat

Posted: Sun May 12, 2013 1:53 pm
by mtngun
Today I tested 2 experimental 120 grain bullets, we'll call them 7EL3 and 7EL4. As it turned out, I mis-guessed on the design of 7EL3 such that it had to be seated well into the powder space. I'm not opposed to deep seating per se, but powder space is an issue with the compressed load that I am using.

So ...... I revised the design so that it could be seated nearly flush with the bottom of the case neck. Both 7EL3 and 7EL4 have some "tricks" that are not obvious to the naked eye and we'll keep it a secret for now.

7EL3 was used in a gas check shootout as discussed in this thread: ... f=11&t=409

Then I cleaned the barrel and shot five 3-shot groups with 7EL4. 7EL4 averaged 2.12" vs. 2.71" for 7EL3, however, the difference was not statistically significant.

Velocity with 7EL3, for both the Hornady check and the Sage check. Not much exciting going on in this chart other than the first shots from a clean barrel being a little slow. Next time I'll shoot a fouling shot.

The 7EL4 shot the best group of the day, sub-MOA at an average of 2693 fps for this string. If I could get it to do this every time, I'd be pretty happy. 8-)

Deets on loads:

42 grain WW760 for 7EL3, 42.2 grains for 7EL4
COL 2.52" for 7EL3, 2.585" for 7EL4
HVR lube
heat treated WW & Sn alloy
both pretty close to 120 grains
both seated to kiss the rifling
2646 fps for 7EL3, 2675 fps for 7EL4
excessive sprue bumps filed off by hand
checks seated in a seating die

A trace for 7EL3:

And a trace for 7EL4:

--7EL4 seems to be moving things in the right direction. There are several design tweaks I'd like to try, but I'm not expecting any major breakthroughs in the design department from here on out. There's only so many tricks you can do to a 120 grain bullet.

--the TC trigger needs more tinkering. It used to be too light and unreliable so I increased the engagement, now it is too heavy for my liking.

--the TC needs a wider forearm for benchrest shooting. I'll try to make one. It doesn't need to be pretty since it will only be used at the bench.

--next time I'll shoot a fouling shot after cleaning the barrel. The first shot in a clean barrel seems to be a little slow.

--I'd like to experiment with a few more lubes

--I might try a different primer. I've never been a fan of Winchester primers, but somehow I ended up with a bunch of them. :lol: However, this is supposed to be a test of cast bullets, not a test of primers, so I don't want to spend a lot of time on different primers and powders.

--I might make a die for filling off sprue bumps. I can get the worst of it by hand, but if I had a hardened, adjustable die, then I could file the base perfectly flat.

That's all I can think of at the moment. :geek:

Re: Measuring a 7-30 Waters throat

Posted: Sun May 26, 2013 6:54 pm
by mtngun
Today's experiment with 7EL6 and an increased powder charge was disappointing. I think I am pushing it too hard and compressing the powder too much. So next time I will drop the bullet weight down to 110 or 115 grains to hopefully achieve the desired velocity without excessive pressure.


Full deets on today's range session in the lube shootout thread: ... 1586#p1586