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0.313" throat experiment

Posted: Tue Feb 19, 2008 2:04 pm
by mtngun
from the old forum:

Since my hi-vel 30-06 loads prefer 0.314" cast bullets, it seemed logical to give the barral a 0.314" throat. No one has ever tried this before, so it was a wild shot in the dark. Inquiring minds have to know.

For $60 plus $9 shipping, Dave Manson supplied a custom throating reamer. Since Dave seems to claim a 0.0005" tolerance, I specified 0.3135" - 0.3140" freebore and 0.2990" - 0.2995" pilot. I didn't specify the angle, but his standard leade angle is 3°.

The actual dimensions are 0.3133" freebore and 0.2987" pilot. It's hard to measure the angle, but it appears to be at least 3° and maybe more. Since this experiment was just a shot in the dark, the slightly undersize dimensions weren't critical.

It would have been easier and faster for me to unscrew the barrel and put it in the lathe to cut the throat, but I elected to cut the throat by hand to show that it can be done.

An extension is needed. You can buy extensions but I made my own. I cheated and used the lathe to make a coupling. The coupling diameter is a perfect fit in the Remington receiver so it supports and guides the reamer from the rear (alternatively, I could have drilled and reamed the primer hole in a cartridge case, and used that as a guide). With the front of the reamer guided by the pilot and the rear guided by the coupling, it was easy to make a straight cut.

The hard part is cutting to the correct depth. I used the upset slug method to make a chamber impression, and then by eye adjusted the seating depth of the reamer so that enough reamer was sticking out of the coupling to clean up the existing throat. It only took a couple of seconds to make the cut.

Then I made another chamber impression. It looked like I needed to cut just a wee bit deeper, so used calipers to adjust the reamer depth, and made another cut. Yet another chamber impression confirmed that the throat was deep enough. In fact, I may have gone a little farther than necessary, but that\s OK for this experiment.

Cutting took a few seconds. Making the coupling and the several chamber impressions took half a day.

The resulting freebore was 0.3133" - 0.3135" diameter, about the same as the reamer.

After considerable trial and error I ended up with a push-thru sizing die that sized air-cooled WW to 0.3127" - 0.3131". Any bigger, and they would not chamber easily. The bullets were oven treated and lubed with Rooster HVR. These bullets were 1-diameter except for a tiny step on the nose.

I loaded 10 rounds each of several different powders. The bullets were seated out to where the front band nudged the tapered lead. At the range, most rounds chambered with moderate resistance. A couple required some cussing, and a couple chambered with no effort. On the whole, that's better consistency than I was able to achieve with the old 2-diameter bullets in the old tapered throat.

At this time of year our private range is opened to the public for sighting in, and it's very crowded. The only available bench was a wobbly wooden left hander that put the rifle a good six inches higher than my shoulder and the morning sun directly in my eye. It was either that or else wait until next year. In retrospect, I should have waited. :cry:

Accuracy with the new throat was 6 - 8 MOA. The uncomfortable bench sure didn't help, but I can't blame it all on the bench. None of the loads even came close to shooting a group. OK, so the 0.313" throat sure didn't improve accuracy. Why, I can't say.

I'm confident that the throat was cut straight, smooth, and concentric. The leade angle may be a little steeper than ideal but I've seen plenty worse. For whatever reason, an oversize 1-diameter throat is not the ticket.

So what is the ticket? I don't know. And since I don't know, I think I should continue using these cheap take-off barrels as guinea pigs for chamber experiments before I shell out big money for a custom barrel and custom reamer.

Plan "B" is to set this barrel back an inch and rechamber it with a tight neck. I think that a tight neck will eliminate the need for the oversize cast bullets, so then I'll be able to focus on the throat length and angle. I'll start out with a throatless chamber, and gradually cut the throat a little at a time. I may be able to salvage my 0.313" throater by regrinding it to 0.309" for the next round of experiments. The rechambering experiment will involve some new (for me) skills and tools so it'll probably take all winter or even longer.

Today's experiments did comfirm that Reloader 19 and Ramshot Hunter are very spikey. Even the usually low-spike WW760 had moderate spikes today -- I don't know if that is due to the different throat or due to the different strain gage. Since the adhesive had failed on the previous gage, a new gage was installed with Devcon "Flex" superglue. Other than the cable coming loose during the RL19 string -- more masking tape fixed the cable -- today's pressures seemed consistent and reasonable. Primary pressures were generally lower than book but so were velocities.

This load with Ramshot Hunter was too hot.
Even WW760 had significant spikes today. Velocities were consistent so I can't explain the noisy pressures.
Very consistent pressures with Ramshot Big Game.
The jacketed validation load -- low pressures but low velocities, too.

Re: 0.313" throat experiment

Posted: Tue Feb 19, 2008 2:46 pm
by mtngun
Some experiments aimed at understanding why the 0.313 throat doesn't work well.

All of today's cast loads used:
60.5 gr. RL 19
Fed 210 primer
180 gr. loverin, Rooster HVR, seated to nudge the leade

The strain gage died today while doing the filler experiments, but fortunately I already had a backup gage installed. The backup gage is the same gage that failed earlier due to the loctite 4211 that would not stick to plastic. That gage had popped off so easily that it seemed to be worth reinstalling. Today's primary pressure with the reinstalled gage was 47,197 psi vs. about 48,000 with last week's gage, so the reinstalled gage seems fine except for a little static.

The theory behind today's experiments goes like this: The fat base helps to seal the sloppy neck, and that's good. But the fat bullets distort as they squeeze down to 0.308", and that's bad. The 0.314" x 0.308" bullets worked better than anything else in the original throat because they were the optimal trade-off between gas seal and bullet distortion. The original throat had a 1.5° leade angle that minimized the distortion. The 0.313" throat has a 3°+ leade angle that increases distortion. The old 2-diameter or tapered bullets only had to distort at the base, but the new 1-diameter bullets have to distort the full length.

First up, some of the 0.314" x 0.308" tapered loverins leftover from the original throat. According to my distortion theory, these should shoot better because only the bottom half of the bullet is being distorted as it squeezes into the leade. And indeed, their 3.68" MOA average is the best full power load to date in the 0.313" throat.

Next up, 0.313" 1-diameter bullets just like last week except I scraped the lube out of every other lube groove. The theory was that the diplaced metal could be pushed into the empty lube grooves rather than distorting the entire bullet. The first group was 13" so I didn't bother shooting the other 5 rounds. Funny, I had tried the every other groove test two years ago in the original barrel and it didn't seem to make any difference at that time. No explanation for today's 13" group, except that the lube does seem to matter.

Next, more 1-diameter 0.313" bullets, except air-cooled. My good friend Bass Ackwards suggested that air cooled WW might do better in the oversize throat, so hey, I'll try anything once. Actually, I was confident that the soft bullets would miss the target, and then I could tell Bass "I told you so", but the 6.25" MOA average was no worse than last week's performance with heat treated wheelweight. Velocity started out at 2715 and gradually deteriorated to 2656, a symptom of barrel fouling, and the barrel did look seriously fouled, but even so I have to admit that the soft bullets shot much better than I expected.

Lessons learned & what's next? Even though the 0.313" throat seems to be a failure, I want to learn as much as I can from it, so ....... next time I will try a 0.313" x 0.309" 2-diameter bullet that is only fat at the bottom 1/4".

...... I'd like to try a bigger GtoB, too. The rifle seems to want more lube plus bigger grooves may reduce distortion as the oversized bullet gets squeezed in the leade.

....... I need to do an autopsy on the failed gage and order some hi-tech epoxy. New gages are on backorder. I have to wonder if my spikey loads are contributing to the early demise of the gages?

Re: 0.313" throat experiment

Posted: Tue Feb 19, 2008 2:56 pm
by mtngun
Update on throat angle: as best I can measure, my Manson 0.313" throater has a 4.4° included angle (2.2° per side).

Re: 0.313" throat experiment

Posted: Wed Feb 20, 2008 8:01 pm
by mtngun
It'll be a while before I receive the Shaw 1-12" barrel and get it installed so in the meantime I'll continue to dink around with the 0.313" throat.

All of today's loads used:

180 gr. Loverin
1.5 GtoB
Heat treated wheelweight or heat treated mystery metal
54.5 gr. WW760
CCI #250 magnum primer
seated to nudge the leade
two each 5-shot groups per load
Note:if I were serious about proving anything I would have to shoot at least three 5-shot groups per load and preferably more. I was just "dinking around" today.

[table][mrow] diameter [col] lube [col]avg 5 shot MOA[col]avg FPS[col]std dev[col]avg KSI[col]avg 2nd KSI
[row].314" x .308"[col]HVR[col]2.4[col]2640[col]0.78%[col]50.6[col]42.6
[row].313" [col]HVR[col]2.4[col]2656[col]0.97%[col]51.9[col]43.6
[row].314" x .308[col]LBT Blue[col]7.8[col]2598[col]1.55%[col]49.5[col]45.0
[row].314" x .308[col]Permatex #2[col]9.8[col]2665[col]0.63%[col]52.8[col]48.7[/table]
Not bad considering the wind was gusting to 40 mph.....


Lubes LBT lube doubled groups, doubled standard deviation, and gave up 50 fps compared to HVR, but other than that, what's not to like? I'll stick with HVR until something better comes along, thank you very much. Permatex #2 (the black tarry stuff) increased velocity 25 fps but accuracy was wild. None of the lubes left enough fouling to complain about, at least, nothing visible to the naked eye.

Diameter The 1-diameter 0.313" (actually 0.3127") bullets shot just as well as the 0.314" x 0.308" (actually 0.3135" x 0.3085") bullets. An earlier test seemed to show that 0.313" shot much worse than 0.314" x 0.308", however, that test involved two slightly different bullet designs, with the 0.313" bullet having a 0.7 GtoB. In hindsight, the difference in accuracy was probably due to the GtoB and not the diameter.

0.313" Throat It didn't improve accuracy like I hoped, but it didn't hurt accuracy either. With its favorite bullet, the 0.313" throat turns in 2.4 MOA, which is as well as the original throat ever shot. The 0.313" throat did lower velocity and pressure to the tune of 50 fps and 5000 psi with most loads.

Jacketed Bullets I haven't been keeping records on jacketed bullet accuracy, but the original barrel would usually put three of the jacketed validation loads into 3/4 MOA. With the 0.313" throat, a three shot jacketed group is rarely less than 2.5 MOA. The oversize throat definitely hurt the jacketed accuracy.

Re: 0.313" throat experiment

Posted: Thu Feb 21, 2008 7:00 pm
by mtngun
I'm still dinking around with the 0.313" throat until the new barrel arrives ............

All of today's loads used the 1.5 GtoB 180 gr. loverin, 0.314" x 0.308" unless otherwise noted, HTWW, Rooster HVR, WW760 powder, CCI #250 primers, all seated to nudge the leade. Only 10 shots per load, not enough to prove anything.

The original throat seemed to shoot best at around 2750 fps, so I am gradually increasing the powder charge, hoping to reach 2750 at safe pressures.
[mrow]charge[col]variable[col]fps[col]std dev[col]psi[col]2nd psi[col]5 shot MOA
[row]54.5 gr.[col]0.5cc 2:1 soda:buffer[col]2661[col]0.96%[col]56,638[col]34,300[col]5.48
[row]55.0 gr.[col]0.316" x 0.308"[col]2692[col]0.69%[col]53,717[col]37,000[col]2.9
[row]55.0 gr.[col] --[col]2655[col]1.49%[col]49,338[col]42,300[col]3.1
The buffer increased pressure out of proportion to the small increase in velocity, and accuracy was below par. The soda buffer did lower the secondary pressure by about 8000 psi. While it's interesting to observe how the soda effects the secondary pressure, I don't plan to experiment with it any further. The buffer creates too many problems and the secondary pressure is not a huge concern with WW760 powder.

The 0.316" base increased velocity, but it increased pressure, too. The secondary pressure was reduced by 5000 psi compared to a 0.314" base. Accuracy was no worse than anything else.

Pressures and velocities are still low, so I might load a few with a little more powder, and maybe try a few 0.316" x 0.313" bullets.