7mm BR

[mtngun]

Continuing testing of H4895 and the 100 gr. GC. Groups numbered in the order that they were shot.

#1 & #2: the H4895 loads already discussed in the previous post. They showed some potential so I wanted to see if it was repeatable.

#3: H4895, with the powder charge upped slightly hoping to tune out vertical stringing. Well, something weird happened -- shots #1-4 formed a decent group, and shots #5-10 formed a tight group, but it was a separate group! Something shifted. I suspect the shift had something to do with the rifle or the rest (I adjusted the sand in the front sandbag about the time the shift occurred), not the load, but who knows for sure?

#4: the first 9 shots were in a respectable 1.4", then the 10th shot was a flier low and right. Frustrating.

#5: 130 gr. bevel base coated with Hi-Tek (photo further down in this post). J.R. brand reclaimed shot that had been quenched after the final cure at 365F -- I did not measure the BHN but based on past experience it might be 18 BHN. Quickload says about 51,000 psi for this load!

#6 - #8: same as #5 but air-cooled. I did not measure the BHN but past experience suggests 12 - 14 BHN. I was expecting the 51,000 psi to destroy these soft bullets, but to my surprise they shot better than the quenched bullets! The velocity variation was better, too.

On #7, one cartridge was such a tight fit that I had to pound the bolt closed with my fist (I dared not extract it for fear that it would spill powder). Its velocity was abnormal at 2843 fps and the POI was out of the group, so I did not count it. In hindsight the bullet probably missed a sizing step.


Here is the 130 gr. bevel base. Sorry for the lousy photo.


Here is a poor quality borecam photo near the muzzle after shooting the uncoated GC's at 2904 fps. There is a little grey wash in the corner of the grooves but nothing serious. This is the worst spot in the barrel, the rest of the barrel was cleaner than this.

[mtngun]

One of my goals for this switchbarrel project is to learn better ways to ensure repeatability of all load variables, so that once I find an accurate load I'll be able to duplicate it on demand. One of the variables that is tough to duplicate on command is amount of "jam" when the cartridge is chambered. More so for cast bullets than jacketed because I am nose-sizing my cast bullets to fit the throat, and the amount of nose-sizing affects the "jam" point. One needs to re-establish the jam point for each batch of bullets.

On the 30BR thread I posted a COL gage based on a dummy case that is pushed into the chamber, and I also made a "dummy case gage" for this 7BR. It works, but there's a fair amount of "feel" involved, so it's not 100% accurate or 100% repeatable.

Here's a different way to measure the jam point, with the hope that it might be more accurate. This picture shows it being "zeroed" against the bolt face. The bolt is closed, the cleaning rod is pushed snugly up to the bolt face, and the sliding lock collar is then locked to the cleaning rod with setscrews.

To prevent the cleaning rod from damaging the muzzle crown, a snug fitting guide has been clamped to the muzzle.


At the other end of the rod there is a brass fitting that has been machined flat.


Then a bullet is pushed into the throat until it jams. The cleaning rod is gently pushed against the bullet point, and the protrusion is measured at the lock collar. If we've done everything right, this protrusion should be exactly equal to the COL at the jam point.


I tried measuring 4 different bullets with both gages, results below. The rod gage always produced a shorter COL than the dummy case gage. Which is right? Well, the rod gage is a direct reading of the bullet point relative to the bolt face, while the dummy case gage is an indirect reading -- it measures the position of the base of the bullet relative to the dummy case, which may or may not be 100% seated in the chamber. I have more confidence in the rod gage method because there are fewer things to go wrong and because less "feel" is required.


The COL gage doesn't tell you where the most accurate COL is, only where the jam point is. Shooting experiments are required to find the sweet spot relative to the jam point. Once you know the sweet spot, the COL gage allows you to duplicate it from one loading session to the next. That's the idea, anyway.

[mtngun]

Trying a new plain base bullet and a new-to-me powder coating.

Hybrid coating seems to be much more forgiving of overcure than polyester coatings, and that's important if you need to heat treat the bullets. I've been mostly content with the off-white hybrid powder coating that I've been using, except that it's not very attractive. Hybrids are normally only available in dull colors. My hope was that this "copper" hybrid coating would provide an attractive copper finish while maintaining the physical characteristics of a hybrid coating. Well, as you can see it doesn't look coppery at all, instead it looks butt ugly. Oh well, as long as I have the bullets made up I might as well shoot them.


Here's the results, with targets numbered in the order they were shot. In general, this barrel did not care for the 115gr. bevel base no matter the coating. The best it could do was 2 MOA, so I will give up on the 115 gr. BB.

#1- 115 gr. BB, hi-tek coating, 31.5 gr. WC844, seated at jam+0 as measured with the rod method.

#2- same as #1 but powder increased to 32.0 gr. and jam+8.

#3- same as #2 but jam+15

#4- same as #2 but jam+10

#5- 115 gr. BB, ugly hybrid PC cured 15 minutes, 32.0 gr WC844, jam+10.

#6- same as #5 except 20 minute cure

#7- same as #5 except 25 minute cure

#8- same as #5 except 30 minute cure

#9- 130 gr. BB, 2 coats hi-tek cured 350F for an unspecified time, air-cooled WW, 32.5 gr. WC844, jam+10. These were an old batch of bullets from several months ago, and some of the coating was rubbing off when the bullets were sized. I suspect the coating was not cured completely, causing poor adhesion and poor accuracy.

#10 & #11- same as #9 except a different batch of bullets with 4 thin coats hi-tek cured 365F for 20 minutes, and air-cooled J.R. brand reclaimed shot (normally 12 BHN). Good adhesion.


The so-called copper hybrid PC seemed to shoot best if cured 25 - 30 minutes, similar to the almond hybrid PC. A 30 minute cure time works out well if you need to heat treat after curing (so far my plain base bullets prefer to be air-cooled).


Conclusions, And Things To Try Next Time And Down The Road:
-- this barrel does not like the 115 gr. BB.
-- This barrel does like the 130 gr. BB, as long as the BHN is right and the coating is good. Including last week's groups: 1.40", 1.35", 1.80", 1.95", 1.20". Average-to-date=1.54". That's pretty decent for a plain base at 2700+fps!
-- I could tweak the 130 BB load, but IMHO it's already pretty close to optimal.
-- how 'bout a 145 gr. BB?

[mtngun]

Today I tested a new 140 gr. BB bullet, and also experimented with different powders.

Here is the bevel base family.


Here is the 140 after nose sizing. In hindsight, I'm concerned that it may have ended up with too many bore riding bands and not enough groove diameter bands, more on that later. FYI the bore diameter on this barrel is 0.275" and the groove is 0.283". Alloy was air-cooled J.R. brand reclaimed shot (typically 12 BHN), 3 or 4 coats Hi-Tek cured 365F for 20 minutes, total coating thickness 0.001". Rooster HVR lube.


I used Quickload to screen powders and estimate charges that would reach my self-imposed velocity goal of 2700 fps. I don't have all of these powders on hand, though. Note that today's loads will be hitting 53 ksi - 60 ksi. WW748 had the lowest peak pressure, which would be a good thing according to the Colonel Harrison school of cast bullets.


H4895 looked good in Quickload but didn't do well in real life. Neither did Ol' Reliable WC844.


WC845 was decent. I purchased WC845 with the hope that it was the military equivalent to WW748, but now that I have worked with WC845 a bit I think it is its own powder, and may not have a civilian counterpart. It seems poorly suited to reduced loads, but begins to shine at full throttle.


WW748 seems well suited to the 7BR, but it is difficult to ignite and doesn't seem to like reduced loads. I used a CCI 450 primer with WW748 and WC845.


I also shot a few rounds with IMR4166 just to get a feel for the burning rate, since it is not in Quickload yet. Here is the burning rate for today's powders from slowest to fastest:
-- WW748 @77.5 fps/gr.
-- IMR4166 @80.2 fps/gr.
-- H4895 @81.3 fps/gr.
-- WC844 @82.2 fps/gr.
-- WC845 @82.8 fps/gr

Conclusions and Observations:
-- the 140 does not seem to be as accurate as the 130.
-- the 140 likes slower powders like WW748 and WC845.
-- in fairness, the nose sizing may not have been optimal on the 140. The bore riding nose may not be the best choice at 2700+fps.

Things To Try Next Time And Down The Road:
-- try the 140 with less nose-sizing.
-- keep tweaking the 130. Try upping the BHN to 14. Try the slower powders like WW748 and IMR4166.

[mtngun]

Trying a new-to-me hybrid PC. The hope here is to find a PC that has the long, forgiving cure time of a hybrid PC, yet the prettier look of a polyester PC. There aren't very many pretty hybrid coatings, plus hybrids tend to be thin and splotchy when applied by tumbling.

Not as perfect as a sprayed-on polyester PC, but I like the appearance much better than the previous hybrid PC's that I have tried. The single tumbled coat averaged 0.0013" thick and 0.0008" out-of-roundness.


A best-guess load was chosen without bothering to test or optimize the load, and used to test the coating at different cure times. As per my habit, HVR lube was used in addition to the coating. A single 10-shot group was fired for each cure time. Obviously a single good 10-shot group cannot prove that a particular cure time is good, but a single bad 10-shot group can prove that a particular cure time is bad. Hence the expectation was merely to weed out any obviously inferior cure times, and to observe any general trend. Shooting results below. "MR" refers to the Mean Radius as calculated with Taran.

Not much difference in group sizes but for what it is worth the 25 minute cure had the smallest mean radius.

The "s" on the cure times is a typo, obviously the cure times were actually minutes, not seconds.


The data in this chart was shot with different loads, different bullets, and even with different calibers, so do not use this chart to compare absolute values. The intent is merely to observe trends. To my eye, the trend for hybrid coatings is that cure time is optimal around 25 - 30 minutes.


At the moment this fire-red hybrid coating is my first choice because I like the appearance, and because its 25 - 30 minute cure time is forgiving, and compatible with heat treating if the need arises.

[mtngun]

In September I was too busy to post some of my shooting results but the shorter version is that I was still having a hard time getting consistent results with coated bullets. On some days with some batches, high velocity plain base would do 1.5 - 2.0 MOA, while on other days 3 - 5 MOA was common. A given batch of bullets seemed to be consistently good or consistently bad, so for now I'm going to assume the variation has something to do with a particular batch of coated bullets.

I've been doing a good job, if I do say so, of eliminating variation in nose-sizing, in seating depth, BHN, and in load techniques in general. That leaves the coating as the suspicious variable.

One way to rule out the coating is to shoot uncoated bullets. Of course that isn't possible with high velocity plain base, but it's certainly possible with gas check bullets. So just as a sanity check, let's shoot a few uncoated GC bullets to make sure the rifle and load are performing decently. This was a randomly chosen powder charge that may not be optimized. It didn't break any new world records, but was "decent" even at 3000 fps. So the rifle and the load seem to be "decent."


I've already done several cure time optimization tests for powder coat, now let's try it for Hi-Tek. These experimental bullet were given a single coat of Hi-Tek 0.001" thick. That required a thicker than usual mix. The thick coating was slow to air-dry, lumpy, and uneven, which is why I normally use 3 thin coats instead of one thick coat. But a single 0.001" thick coat made more sense for this cure time test.

As always with my cure time tests, I only shot one 10-shot group per cure time, not enough to prove that a load is good. But one lousy group may be enough to prove that a coating has failed. My goal was merely to weed out any obviously failed cure times.

I used a similar load of WC845 and 130 gr. BB as used in the Fire Red powder coat test, but I upped the powder charge to bring the velocity up to 2700+ fps. Also I used air cooled WW at ~11 BHN. So it's not exactly the same load as the powder coat test and not an apples to apples comparison to the powder coat test.

As you can see, none of the Hi-Tek groups were very accurate. On the other hand, none of them had severely wild fliers indicative of coating failure, either.


The Hi-Tek results graphed. FWIW, the 15 minute cure at 365 F shot as well as anything. None of the test bullets experienced flaking while sizing.


Conclusions and Observations:
-- 15 minutes at 365 F seems adequate for Hi-Tek.
-- But if you apply 3 coats and cure each coat for 15 minutes, the 1st coat will have 45 minutes total cure. Is that optimal? Perhaps I should reduce the cure time for the 1st and 2nd coats?
-- WC845 does not burn well in reduced loads. Nonetheless I have 2 jugs so that's why I'm using it.

Things To Try Next Time and Down The Road:
-- experiment with 3 thin coats Hi-Tek, perhaps 12 min + 12 min + 15 min at 365 F ???
-- but mostly I'm inclined to abandon Hi-Tek and use the Fire Red Hybrid. Why? Because the Fire Red Hybrid shoots just as well and is much faster to apply. Three coats of Hi-Tek requires the better part of a day, mainly due to the air-drying time for each coat.
-- keep playing with the 100 gr. GC at 3000 fps. I was surprised that it shot as well as it did at that velocity.

[mtngun]

Today's plan was to shoot more of the 100 gr. GC at 3000 fps to see if it was repeatable. To be sure that the velocity would be 3000+ fps -- because "cast bullets at 3000 fps" has a nice ring to it -- I increased yesterday's powder charge by 0.1 grain. But as usual, Mr. Murphy intervened. It was 70 degrees when I shot yesterday's load, but this morning was below freezing. It was 50 - 60 F in the shooting shack, but even so velocities fell short of 3000 fps. Drats! Apparently WC845 is very sensitive to temperature.

Anyway, here is the bullet, loaded for 0.015" jam, if I remember right.


And here is the target. The top 2 groups were shot yesterday, the bottom 3 groups were shot today. Not exceptional, but decent, especially considering the velocity and the uncooperative powder which had more velocity variation (0.9% typical) than I prefer.


Conclusions and Observations:
-- this barrel likes this bullet.
-- the 3000 fps load would be great for testing lubes, and I do need to do a lube test because I'm almost out of Rooster HVR.
-- I'm not sold on WC845, which seems to require a full throttle load to burn decently.

[mtngun]

As discussed in another thread, I'm preparing to do an alloy shootout, probably with the 7BR since it has been more reliable than my other barrels. Just for variety I'd like to use a different load than the 100 gr. @ 3000 fps that I used in the lube shootout. Should I increase the velocity to 3150 fps? Should I try a powder that burns better than WC845? Should I try a heavier GC bullet?

Today I sampled different powder charges and a different bullet, just enough to get a feel for what is possible. All loads used BAC lube and oven treated reclaimed shot. The bullets were nose-sized to match the throat and seated for 0.010" - 0.015" jam.

Groups were shot from left to right:
#1) the usual 100 gr. GC
35.5 gr. WC845
CCI #450
3124 fps
93 ES
0.76% velocity standard deviation

#2) same as #1 except powder increased to 35.9 gr. WC845
3163 fps
65 ES
0.71% velocity standard deviation

#3) a 118 gr. GC
35.0 WC845
CCI #450
2991 fps
75 ES
0.75% velocity standard deviation

#4) 118 gr. GC
33.0 LT32 (a drop tube was used to make the powder fit)
Rem #7 1/2
3000 fps
32 ES
0.32% velocity standard deviation

#5) 100 gr. GC
33.0 LT32
Rem #7 1/2
3154 fps
23 ES
0.27% velocity standard deviation


A close up of the 118 gr. bullet. It was kind of an accidental design but it seems to shoot well enough.


Conclusions:
-- yes, the 100 gr. bullet can be safely pushed to 3150 fps with either WC845 or LT32.
-- LT32 has much better velocity variation.
-- the 118 gr. bullet hinted at better accuracy, though I didn't shoot enough to prove anything.

I'm leaning toward choosing the 118 gr. or something similar, pushed to 3000 fps with LT32. It seemed like it wanted to shoot. At any rate, I just want to use something different in order to gain more experiences and knowledge.

[mtngun]

Still in search of a new and exciting load for the alloy shootout ... all of today's loads used oven treated J.R. reclaimed shot, BAC lube, and a Remington 7 1/2 primer.

#1) 100 gr. GC., 33.0 gr. LT32. A repeat of the load that shot MOA last week -- today one flier spoiled it! ES was 41 fps, or 0.41%.

#2) 115 gr. GC., 33.0 gr. LT32. ES was 28 fps, or 0.29%. Despite the excellent ES, this load clearly had vertical dispersion, presumably due to unhappy barrel harmonics. Sorry, I forgot to take a photo of the bullet, but it's a stretched version of the 100 gr..

#3) 115 gr. GC, 33.4 gr. LT32. This was an attempt to find a happier vibration node, but it didn't seem to help. ES 41 fps or 0.44%.

#4) 115 gr. GC, 32.3 gr. LT30. Another attempt to find a happier vibration node, and it did seem to get rid of the vertical, but only ho-hum accuracy. 32 ES or 0.29%.

#5) 100 gr. GC, 32.3 gr. LT30. Prolly merely a lucky group, but I'll take it! ES 32 fps or 0.36%.


Closeup of today's lucky group. Groups like this provide some consolation for my many experiments that don't work well.


Conclusions:
-- the 100 gr. & LT32 combo acted like it wanted to shoot but darned that flier! In fairness, it is to be expected that fliers will happen more often as I walk up the RPM's. 3150 fps in a 14" twist = 162,000 RPM.

-- the 115 gr. clearly had bad barrel harmonics with LT32 at 3000 fps. There was no room for more powder to try a significantly higher velocity, and I'm just not interested in lower velocities at the moment. As time and money allow I would like to retest this bullet with more powders.

-- the 100 gr. & LT30 showed potential -- but one group doesn't prove much.

-- I'm eager to move on to the alloy shootout, so for the time being I'm going to stick with the "ol' reliable" 100 gr. GC and give LT30 a chance to prove its repeatability.