CBA loads

[mtngun]

I stuck the barrel back in the lathe and re-cut the throat. I only cut deep enough to make contact all the way around the throat, and that ended up making the throat about 0.030" longer than before. The idea was to clean up any burrs or any uneven-ness, even though I could not see any burrs or any uneven-ness.

I never did remove all the lead from the throat. In addition to the Birchwood Casey lead removal cloth, I tried JB paste, and fine steel wool wrapped around a brass brush. In the end only about half of the lead was removed from the throat.

Then I fired 5 firelapping rounds with #500 grit. The idea was to knock off any microscopic burrs in the throat -- the rest of the barrel does not need to be lapped. That brings the total firelapping rounds for this barrel up to 25 (10 #280 and 15 #500).

The original batch of cases was set aside because the necks had been turned too close to the shoulder and they were prone to separating at the neck/shoulder junction. A new batch was made and this time I turned the necks a little thinner, aiming for 0.0105". Loaded necks ran around 0.331" in the 0.332" chamber.

Now continuing the search for Mr. Happy Powder ...

I meant to use magnum primers with Accurate 2520, but unintentionally loaded it with Federal #210 standard primers. The ES was pretty lousy so apparently AC2520 does prefer a magnum primer, plus it would probably do better at higher pressures.

I'm finding that this chamber gives velocities about 100 fps faster than Quickload predicts, hence most of these experimental loads are faster than the 2300 fps goal.

As you can see, IMR3031 made a shotgun pattern. N135 was the only powder that even tried to group today.


Showing the "problem" land at the entrance to the throat, taken with a Lyman borecam. As you can see there was still lead in that spot at the beginning of the test, and the amount of lead stayed about the same. Next time I may try shooting a few jacket bullets to knock the lead out, since I have already tried everything else.


All of today's powders left some carbon, and some lead in the corners of the grooves, particularly in the middle of the barrel.


Well, this chamber & barrel combo continues to be a dog. I have lots of questions but not many answers. There are 2 more powders to try before I give up.

[mtngun]

I fired 3 jacketed bullets hoping to knock out the lead in the throat. It may have removed some of the lead, but not all of it. Maybe if I fired 100 jacketed bullets, then all the lead would finally disappear?

Then continuing the search for Mr. Happy Powder .....

Both Varget and IMR8208 made shotgun patterns. Varget had a respectable ES, while 8208 had a lousy ES.


Both Varget and IMR8208 left a little leading in the corners of the grooves at mid-barrel. The darkness of the photos is not the natural color, it's just that I edited the lighting in order to improve the contrast and make the leading easier to see.


Here's the "problem" land in the throat:
Top Left: you can see a little copper wash left over from shooting the 3 jacketed bullets. Is that a pit on the land?
Top Right: it still looks like a pit on the land
Bottom Left: it still looks like a pit on the land.
Bottom Right: same land but closer to the throat entrance. It looks like a chunk of the land is missing?

The leading seems to be gradually diminishing as more shots are fired, and we're able to get a better look at the land. It appears that the "problem" land has some minor damage -- possibly pits, or possibly tears. Perhaps the throating reamer grabbed the land and tore a chunk out instead of cutting cleanly? If so that would explain why lead accumulates by that land but not by the other lands.


There are always more powders i could try, but I think I have tried enough to get a feel for what this chamber/barrel combo will do. I'll summarize my thoughts on this chamber in the next post.

[mtngun]

A summary of loads with the 193 gr. bullet, for both the 30BR and the 308i. It's going to take a long time to analyze this data so in the meantime I'll just note the obvious stuff:
-- IMR4350 was the most accurate powder in both 30BR and 308i (though I did not try N135 in the 30BR)
-- obviously the 308i chamber was a dog, though I'm not sure why. More on that later.



Why was the 308i chamber a dog?
-- the "problem" land in the throat that may have a jagged or pitted edge?
-- perhaps simply too much case capacity for a clean, consistent burn?
-- what am I leaving out? It's not like I'm the first person to shoot cast bullets in a 308.

Where do we go from here?
-- let's assume that the powder of choice is either IMR4350 or N135.
-- should I admit defeat at 2300 fps, put my tail between my legs, and settle for 2050 fps like most CBA competitors?
-- or should I try again with a better throat and a more ideal case capacity?

What is the ideal case capacity for 2300 fps with IMR4350 or N135?
-- if you believe Quickload, 48 gr. H2O would give 2300 fps at or near 100% filling load ratio (compare to 41 gr. for 30BR and 56 gr. for 308W).
-- the resulting pressures would be 43,000 - 45,000 psi.
-- the resulting accelerations would be 104,000 - 109,000 G's.
-- hence even with ideal case capacity, pressures and G's would be at a disadvantage compared to normal CBA loads.

[mtngun]

MUSINGS: I've been busy putting out various fires and haven't been able to do any shooting or gunsmithing, and besides I'm still trying to make sense of the terrible 308i results.

In the previous post, I concluded that the 30BR case was a little too small for my application while the 308 case was a little too big, and that the ideal case would hold 48 grains H2O (if you believe Quickload).

48 grains is pretty close to the 30 x 1.75" wildcat that enjoyed some minor popularity during the early IHMSA days. The original IHMSA game used the same "full footed" rams that the rifle silhouette game used. It turned out that many handgun cartridges could not reliably knock over the "full footed" rams. A 308 Winchester was a reliable "ram slammer," but recoil was substantial, so there was experimentation with milder 30 caliber cartridges like the 30 Herret, the 30 x 1.5", and the 30 x 1.75".

One response was to stretch the 357 case to what became known as the 357 Maximum. However, the 357 Maxi (or Super Mag, as IHMSA head Elgin Gates insisted it be called) was still not 100% reliable on full footed rams. Oh dear, Elgin's pet cartridge might not be the ultimate silhouette cartridge after all! Lo and behold, the rules were suddenly changed to use smaller feet on the rams, making them easier to knock over. A 30 caliber cartridge suddenly seemed like overkill, and the 30 x 1.75" was quickly forgotten as competitors shifted to 7mms and even smaller calibers.

(Personally I preferred the full footed rams and thought it made the game more interesting, forcing competitors to use powerful cartridges similar to what they might use for big game hunting, but other competitors, particularly 357 mag shooters, complained loudly when their not-so-powerful loads left a ram standing.)

I formed a couple of 30 x 1.75 cases, one from a Lake City case and one from a 300 Savage case. The Lake City case held 44 grains of H2O while the 300 Savage case held 48 gr. H2O. 44 grains is so close to the 30BR's 41 grains that it does not seem worth the extra effort. I am leaning toward setting the barrel back and rechambering to 30 x 1.75 made from commercial cases. That would kill two birds with one stone -- clean up the jagged land in the throat of the 308i, and provide a case capacity that might be closer to ideal, at least on paper.

[mtngun]

An updated table for "proven cast benchrest loads." It does not include plain base loads, pistol loads, production loads, or military class loads. Generally my criteria for inclusion is a 100 yard 10-shot average of 1.1" or better -- in other words, ten shot MOA. I'll gradually add more data as time allows, but I'm going to start a separate thread for this table since eventually it will expand to more calibers, not just 30 caliber.

The idea is to look for any quantifiable trends that might be part of a "Unified Theory of Cast Bullets." Obviously, there are other important factors that can't be quantified -- the quality of the barrel, the quality of the throat, the bullet design, how cleanly and consistently the powder burns, etc.

[mtngun]

Today I set back the barrel and rechambered it to 308 x 1.75".

Threading the tenon at 160 rpm. I upgraded to an Asian threading tool a while back and really like it. I had worried that carbide would not cut well at slow speeds but if anything it cuts better than my old HSS threading tool.


Double checking alignment after threading. The indicator probe reaches into the barrel and reads directly off the lands and grooves. The alignment had moved a thou or two while threading so I dialed it back in before moving on to reaming. The muzzle end of the barrel was dialed in, too. There are endless internet debates about the best way to dial in the barrel for reaming and I'm not going to settle that debate here.


I worked slowly and carefully so it took most of the day to finish reaming the barrel, but I think everything turned out all right. Tomorrow I'll prep cases and bullets.

By the way, I was reminded that CBA competitor Tom Gray had experimented with stretching the 30BR case for more velocity. His version was essentially a 308 x 1.65", but he called it the 30 Gray. Like me, Tom used a 1/2 degree leade, though his throat had 0.100" freebore while my throat has little to no freebore.

[mtngun]

Forming cases from Winchester brand 308's. The first two cases crinkled their necks in the sizing step. I thought about it and remembered that while forming the dummy cases I had sized the cases very slowly. So on the 3rd case, I sized the case in slow motion, and the neck came out fine, as did the remaining 47 cases.

Then the necks were turned to 0.011" thick to fit in the 0.331" - 0.332" chamber neck.


After prepping the cases, prepping another batch of 193 gr. bullets, and loading 22 rounds, it was nearly dark and I was worn out, so test firing will have to wait until morning.

[mtngun]

Breaking in the new chamber. As per my usual procedure, the barrel was brushed in between groups and one fouling shot was fired at the target. If the fouling shot landed within the group it was counted. Both of today's fouling shots landed inside their group hence we have 11-shot groups.

I misguessed the powder charges so velocities were either too slow or too fast. The good news is that it shot groups, not shotgun patterns, so I was happy after being jinxed by the 308i from hell.


The IMR4350 load was too slow and, at an estimated 36,000 psi, probably did not generate enough pressure to burn well, so the barrel was quite sooty. The reason the photo looks dark and dingy is because the barrel was dark and dingy. There was no leading either in the throat or at mid-barrel.


No leading in the throat after N135 at 48,000 psi. In hindsight, the 308i's problem with leading in its throat was probably due to the defects on that one land.


A wee bit of lead in the corner of the groove at mid-barrel after N135.


Fire-formed cases held 47.5 gr. H2O, close enough to my goal of 48.0 gr.. Perhaps if this chamber turns out to be a good shooter then I may upgrade to Lapua cases with the small primer pocket, but the Winchester cases will suffice for initial trials.

This chamber has a snug 1/2 degree leade. The entrance to the throat is 0.309" and the taper begins immediately, with no straight freebore.

I forgot to gage the chamber neck diameter but a 0.331" reamer was used to cut the neck and it usually ends up 0.332". After turning necks to 0.011", a cartridge loaded with a 0.3085" bullet measures 0.3300" - 0.3305" at the neck, and a fired neck measures 0.331". The gap between the end of the case and the start of the throat is minimal.

Today's target was a decent start, with the N135 load acting like it wanted to shoot despite being hotter than intended. I suspect the IMR4350 load was ho-hum because it did not generate enough pressure to burn cleanly. I will keep tweaking the charges with both powders in search of a sweet spot at 2200 - 2300 fps.

[mtngun]

Yesterday's 4350 load was too slow and too sooty, so I increased the powder charge hoping it would burn better. Today I used Accurate 4350 because I'm nearly out of IMR4350. Even though they generally use the same load data, AC4350 is definitely a different powder -- the kernels are shorter it meters better, and Quickload seems to think it burns more progressively. But how does it work with cast bullets?

The one flier was shot during wicked mirage boil, so the mirage may have been to blame, or then again it could be the load. It would take more than one group to know for sure. Even including the flier, the stiffer charge of AC4350 was a bit more accurate than yesterday's load with IMR4350. It acted like it wanted to shoot, but it's still not generating enough pressure to burn well as evidenced by the 83 ES. The barrel was still sooty, too.


Yesterday's N135 load was too fast, so today I tried lower charges. The 33 gr. target was decent, but what the heck happened at 34.1 gr.? Perhaps 34.1 gr. is a bad vibration node, but if so that's a helluva lot of vibration.


None of today's loads leaded the throat. The throat leading problem that plagued the 308i chamber seems to have been fixed. Apparently the 308i's problem was due to the minor defects on that one land. It's a shame because now I'll never know how the 308i might have performed with a good throat.

The 33 gr. N135 load was very clean other than the usual dark carbon haze.


Things to Try Next Time
-- try 41 gr. AC4350? Easier said than done because it will be heavily compressed.
-- retest N135 at 33.0 gr, 33.5 gr. and 34.0 gr. ? N135 acts like it wants to shoot, just need to find the sweet spot.
-- it may be that 1 MOA is all this particular barrel has to give. I've tried different chambers, different bullets, and lots of different powders, yet I keep running into a glass wall at 1 MOA.
-- while there are always more load variables that could be tweaked, and should be tweaked as time allows, I'm anxious to settle on a powder charge so I can get back to testing bullet designs,

[mtngun]

How do you determine the trim-to length for a particular non-SAAMI chamber?

SAAMI chambers are normally so sloppy that you just pick a common trim-to length and you'll be fine.

But on a benchrest chamber, you want the gap between the end of the case and the start to the throat to be minimal (like 0.010") because the cast bullet is unsupported as it crosses the gap. The gap is also bad for jacketed bullets because it accumulates carbon. OK, so how do you measure that gap?

At times I have used pin gages to determine the location of the throat entrance relative to the breach. That system works well enough on single shots where you have easy access to the breach end of the barrel, but it doesn't work so easily on a bolt gun. On a bolt gun, you can use pin gages to measure the distance between the throat entrance and the end of the barrel, but more measurements and some math are required to calculate the distance between the bolt face and the end of the barrel. On some benchrest actions that distance is standardized and recorded so you don't have to re-calculate it every time you chamber a barrel. I should probably do that with my M700, since I already use standardized dimensions to fit my barrels.

But in the meantime what I've been doing is inserting a bore scope through a modified dummy case to eyeball the gap between the end of the case and the start of the throat. After I ream the chamber and the neck and verify headspace, then I firmly push the dummy case into the chamber and insert a borescope to verify that the gap looks about right. If there is not enough gap then I ream the neck a little bit longer. And that's how I set up the chamber neck length on the 308x1.75".

But ... what if the dummy case does not go all the way in to the chamber, due to friction? Then it might appear that there is sufficient gap between the end of the case and the start of the throat, when there really isn't. I try to cut my chambers very tight, so there may well be some friction that makes it difficult to push the dummy case in all the way with finger pressure. It's not a super precise system.


So where is this conversation leading? Today the 308x1.75 did not want to shoot, even with loads that had shot decently previously. It seemed like something must be wrong, so I started checking all the things that might go wrong.


I had been trimming cases 1.747", and my dummy case was 1.745", yet when I pushed the dummy case into the chamber firmly (using a cleaning rod guide as a pusher) I could not see a gap between the end of the case and the start of the throat. So I gradually trimmed the dummy case a little and tried again -- still no gap. I kept trimming and checking until finally a definite gap appeared at 1.724".

Now, there may be some measurement error for all the usual reasons -- it depends on how hard you push the case into the chamber and on exactly where you position the bore scope, but I'm pretty sure the 1.745" cases had been jamming into the throat. That could definitely play hell with accuracy. Was that why accuracy sucked today? Maybe, maybe not, but the bottom line is that the problem needs to be eliminated.


Either the cases need to be trimmed to 1.725", or else the chamber neck needs to be cut 0.020" - 0.025" longer. Cutting the chamber neck is more work but that's the right way to do it, so I'll schedule the job for this Saturday. Shooting is on hold in the meantime.