Continuing our experiments with polyurethane coating and different BHN .....
First up, repeating the test of uncoated vs. coated gas check at 2700 fps. The usual load of 27.6 gr. WW760, #41 primer, sized 0.246" x 0.243" (or 0.242" for the uncoated bullets), HVR lube on all bullets both coated and uncoated, seated 0.005" away from contact.
Off topic but today I tried backing off the case sizing die as much as I dared to see if that would help with the misfire problems I've been experiencing. Contenders have a wimpy ignition to begin with and it's complicated by rimless cases that headspace on the shoulder. If you size the case for loose headspace then you get misfires because the cartridge moves when hit by the firing pin. If you size the case for tight headspace then the action may not lock up 100% and then you get misfires because the hammer grazes the hammer block interlock. You're damned if you do and damned if you don't.
In any event, I had zero misfires with today's snugger headspace, so maybe I finally found the sweet spot?
Here's the target for the gas check bullets at 2700 fps. To my eye the uncoated bullets look less accurate particularly with regards to horizontal dispersion. The coated bullets are tighter horizontally but plagued by vertical stringing.
But .... the difference between the groups was not statistically significant. Summary of the mean radius for each of today's gas check loads:
-- 0.84" for coated BHN 11 (only 11 shots)
-- 0.86" for coated BHN 45 (33 shots)
-- 0.97" for coated BHN 25 (10 shots)
-- 1.11" for uncoated BHN 25 (22 shots)
Summary of velocity for each of today's gas check loads. The BHN 45 bullets were the fastest because they have more engraving resistance which increases pressure, just the opposite of why most people think some loads are faster than others.
-- 2659 fps for coated BHN 25
-- 2666 fps for uncoated BHN25
-- 2668 fps for coated BHN 11
-- 2723 fps for coated BHN 45
FYI here is the Quickload estimate for the GC load. Quickload is not the Bible but if the predicted velocity matches the actual velocity then I figure Quickload is pretty close on pressure, too. In this case the predicted velocity was within 20 fps.Conclusions & Observations for coated GC bullets:
-- I suspect that coated GC bullets are slightly more accurate than uncoated GC bullets at 2700 fps, but the difference is too small to "prove" in this not-so-accurate load.
-- the bottom line is that coated GC bullets did not hurt
-- my general takeaway is that coating GC bullets may be worthwhile in a "bad" barrel that is leading, but improvements are minimal in a "good" barrel that does not lead. That makes sense when you consider that a coating does pretty much the same thing as a gas check -- it greatly reduces base melting and gas cutting.
-- one good thing about the coated GC bullets is they did not seem fussy about BHN. Dunno if that is a general rule or merely peculiar to this load & this rifle.
-- this load in particular and warm Contender loads in general have a tendency to string vertically. Perhaps tweaking the powder charge would help, perhaps using the same cases instead of mixed cases would help (!!!)
, but I suspect the Contender action flexes in an up and down direction and that is the nature of the beast. We'll see if the vertical stringing goes away when I switch to a bolt gun.
Next let's test coated plain base bullets with a softer alloy. I used a mystery alloy that someone gave me, they told me it was a muzzleloader alloy with 1% tin, but the density and hardness suggest it has a little antimony in it, too. In any event it averages 8.5 BHN.
Here's the target. The soft coated bullets started out poorly and got wilder as more shots were fired, probably due to fouling.
Did the borescope reveal leading after shooting the soft coated bullets? Only a tiny bit. This is just past the throat, to me it looks like there is some crud in the corner of the groove.
This is 10" past the breach, I dunno what the dark spots are but the bottom line is that it's not squeaky clean.
This is 3" from the muzzle and is squeaky clean.
FYI here is the Quickload estimate for this plain base load. In this case the predicted velocity was only off 20 fps so I figure that's pretty good. Conclusions and Observations for Coated Plain Base Bullets:
-- so far best accuracy has been with air cooled wheelweight at approximately BHN 11.
-- BHN 8.5 shot wild. BHN 25 shot even wilder.
-- 3 thin coats of polyurethane shoots as well as any other coating I have tried.
-- coating plain base bullets definitely helps at 2000+ fps. It greatly reduces base melting and gas cutting and allowed me to increase the velocity to 2570 fps at 25,000 psi.
-- however, the 2570 fps load seems to be right on the ragged edge of failure and at the moment I don't have any more tricks up my sleeve to improve it.Things to Try Next Time
-- water based polyurethane. In theory it's the same stuff after it's dried, but water-based may be easier to apply if it dries faster than oil based.
-- as time and money allows I'll test more coatings but at the moment I short on time and money and want to move on to other projects.
-- In the next few days I will post a thread explaining how I apply the shake-n-dry polyurethane, in case you have been wondering.