Measuring a 7-30 Waters throat

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mtngun
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Measuring a 7-30 Waters throat

Post by mtngun » Fri Mar 08, 2013 1:37 pm

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.
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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.
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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.
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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.

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Re: Measuring a 7-30 Waters throat

Post by mtngun » Fri Mar 08, 2013 2:01 pm

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.
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The bore rider.
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Re: Measuring a 7-30 Waters throat

Post by mtngun » Tue Mar 12, 2013 7:16 pm

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.
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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".
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The die pushes the bullets completely through rather than relying on the next bullet to finish the job.
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Then the sized & checked bullets get a trip into the oven at about 435 degrees F for an hour.
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Then the hot bullets get doused with water.
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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.

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Re: Measuring a 7-30 Waters throat

Post by mtngun » Sat Mar 23, 2013 4:17 pm

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.
Image

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Re: Measuring a 7-30 Waters throat

Post by mtngun » Sun Apr 14, 2013 3:04 pm

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."
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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.

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