Cast boolits and chrono numbers

[FergusonTO35]

Hey guys. Has anyone noticed that with some cast boolit loads velocity declines as more boolits are fired? This happens to me every so often. I'll run a particular load through the chrono, starting with a clean gun. As more rounds are fired the average velocity starts to decline, sometimes the extreme spread gets bigger also. It seems most pronounced in short barrels, such as my snubnose .38 and Kel-Tec P32. Anyone ever notice this before? Could the build up of fouling be changing the pressure curve and burning characterisitics? Accuracy seems about the same when this happens.

[Shiloh]

I never run more than two strings on the chrono when clocking rounds.
The ones I run are quite close but different. I attributed it to the gun warming up.
You bring up an interesting point that others may be able to expound on.

When working up loads, I bring several different charges and a notebook. If I fill up the chrono,
I write down the data, erase it and continue.

Shiloh

[geargnasher]

If that happens with a rifle, the lube isn't holding together as the gun gets hot.

Gear

[Larry Gibson]

It happens both in rifle and handgun. The barrel gets seasoned or fouled and that can slow the bullets down. I've also seen velocities increase as as the barrel gets and even coat of a good working lube in it,again in rifles and handguns.

Larry Gibson

[GRid.1569]

It happens both in rifle and handgun. The barrel gets seasoned or fouled and that can slow the bullets down.

Larry Gibson

I note that if I clean my Marlin (squeeky clean with Kroil and degrease) it'll take around 30 rounds to return to the original point of impact... after that it'll stay on the mark 'til the next clean up (many thou's rounds later)....

[beagle]

In a clean barrel, i'd attribute that to barrel seasoning. With no lube, there's slightly more friction and resistance hence more pressure and more velocity. As the barrel becomes fouled and lubed, less resistance and less pressure and velocity.

Darn, wish we had a simple device like a chrony to monitor pressure. We'd learn a lot./beagle

[leadman]

I noticed this many years ago and now just run a bore brush once thru the bore and velocity and accuracy will be restored. I think it is a combination of the lube and powder fouling and possibly primer fouling that causes the changes.

Awhile back I bought one of those 223 pellet conversions from the Sportsman's Guide that uses a shotgun primer. I never realized how much fouling primers make until firing this. After about 4 or 5 shots the pellets were going everywhere but where I aimed them. Previous to this number of shots they were just about going thru the same hole.
When I ran a brush thru the barrel it was extremely hard to push it thru. It took many applications of #9 and a brush to remove the black fouling. I discontinued use of this.

[geargnasher]

I note that if I clean my Marlin (squeeky clean with Kroil and degrease) it'll take around 30 rounds to return to the original point of impact... after that it'll stay on the mark 'til the next clean up (many thou's rounds later)....

Then you're doing everything right, have a balanced load, and a good lube that works with the alloy, barrel, and pressures you use. You have achieved "nirvana". This is one of the reasons I don't like to clean my rifle barrels, they have to re-season afterwards. I use Ed's Red for light cleanings, if you just barely dampen a patch with it it will wipe out the powder fouling and prevent rust for short periods, but isn't so powerful that it soaks out the "season" from a good lube.

If you have to clean every so many shots to restore accuracy, as is often the case with lubes containing Alox or calcium soaps which tend to accumulate, then a lube change is in order. A good lube will maintain consistent bore condition for many hundreds, even thousands of rounds without cleaning.

Gear

[runfiverun]

bet it levels out after about 7- 12 rounds from the clean bbl.

after that if you see inconsistencies it can be attributed to the load [seen as stringing on the target] or your lube getting too wet [giving off too much oil] in the heat.

[FergusonTO35]

I think the responces here are right on the money. Last night I was reviewing my chrono data from last Saturday and I found that, after 8 rounds, the average velocity for my .32 Auto with 2.1 grains of Titegroup declined from around 810 to 765 and then stayed there consistently. I've also found that my pet .38 Special load in my S&W 10-5 with 4" barrel stays around 870 fps regardless of how many rounds are fired. Perhaps the longer barrel gives the powder more time to burn, compensating for the build up of lube and fouling. Accuracy with the 158 grain boolit is always spot on no matter what. Man I love that 40 year old police trade in, best centerfire handgun I've ever owned!! :-)

[blikseme300]

As Gear said.

I found that over cleaning of rifle barrels caused drifting of POI. This is bad, especially when hunting. My opinion is that over zealous cleaning is an artifact and legend perpetuated from when service ammo was corrosive. YMMV


Bliksem
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[fecmech]

My opinion is that over zealous cleaning is an artifact and legend perpetuated from when service ammo was corrosive. YMMV


Amen to that

[GRid.1569]

When I shot pistol.. PPC... (don't get me start on that ... )
I described it as "accurising crud"... LOL....

"100grn'rs don't work at 25meters..."

So those "X's"... are ... What?...