Odd Phenomenon - 38 Special target rounds

[Nick_C_S]

This one is strange, and I hope someone with a lot of smarts can tell me what’s going on.

Dealing with 38 Special, X-treme plated 158gn SWC, Nitro 100 NF powder, Winchester WSP primers, and Starline brass. Only the powder charge varies a bit. The firearm is always the same S&W model 67 with a 4” bbl.

Basically, whenever I chronograph multiple strings of ten, the following string always underperforms the previous – with remarkable consistency, and without exception.

On 4/17, I chronographed three strings of ten. The charge was 3.6gns.

First string: 782 f/s average.
Second string: 768 f/s average.
Third string: 732 f/s average.

I thought it was odd, but passed it off as maybe shading from the range cover was moving onto the chronograph diffuser, or something. Didn’t really dwell on it.

On 5/28, I tested two strings of ten. The charge was 3.5gns.

First string: 751 f/s average.
Second string: 711 f/s average.

At this point, I noticed the correlation with the bullet. No other bullet profile is doing this.

So today (6/30), I tested two strings of ten. The charge was 3.6gns. (It was a different lot than the ones on 4/17 – but otherwise, the same loading.)

First string: 746 f/s average.
Second string: 687 f/s average.

Then I tested six strings of ten. The charge was 3.8gns.

First string: 756 f/s average.
Second string: 717 f/s average.
Third string: 714 f/s average.
Fourth string: 697 f/s average.
Fifth string: 673 f/s average.
Sixth string: 652 f/s average.

Then there was a cease fire. I’m thinking this is really strange. Maybe my chronograph is going wonky? So when the range went hot again, I tested some 45 ACP (Colt 1911 full size) that I had already tested three strings of ten earlier in the day. They were 659, 672, & 662 – nice and consistent (yes, they’re powder puff rounds). So I tested another string of ten – 666 f/s – right on the money.

Then I went back to the 38 and tested some lead SWC’s – Missouri Bullet Co “38 Match.” These are a soft cast BHN-12 bullet. They were 782, 788, & 789 – with excellent SD’s, btw. As you can see, no drop in velocity.
So I decided go to back to the plated ones and run another test with my remaining forty rounds.

First string: 750 f/s average.
Second string: 716 f/s average.
Third string: 675 f/s average.
Fourth string: 659 f/s average.

This is really odd. Also noteworthy was how quickly and consistently the velocities dropped just going through the first string of ten: 839 (the fastest recorded of the day), 792, 791, 787, 763, 765, 695, 699, 697, & 671. So that 750 average would have been much higher if only the first five were counted.

It is obvious that these rounds progressively reduce in velocity for some reason, but I have no idea why. I’ve never seen anything like this. There’s just way too much data for it to be a coincidence. What is going on??

 

[jmorris]

That is pretty strange alright. If it were me I would have kept going until they stopped loosing velocity.

I would have asked what powder but it doesn't seem to be a temperature related problem (100 fps would be huge anyway) but it seemed to go back to "normal" after the cast bullets and start the drop again.

Do the test again and keep going until it reaches a "bottom".

 

[SHR970]

Are you noting lots of copper deposits when you clean the barrel?

This sounds like a build up issue of some sort.

 

[Nick_C_S]

If it were me I would have kept going until they stopped loosing velocity.

Well, I ran out of ammo. Also, some rounds were getting into the low 600 fps range - and that is my self-imposed minimum velocity for plated (or jacketed) bullets in a revolver. So even if I hadn't ran out of ammo, I was starting to get concerned.

Are you noting lots of copper deposits when you clean the barrel? This sounds like a build up issue of some sort.

Haven't cleaned it yet (rare for me, btw; I usually clean right after a range trip, but it's been a busy day; plus, it's literally 100+ degrees in my garage right now - 108 today; it was 90 at the range at 9:30am). But I did just take a look down the barrel. Fortunately, Nitro 100 is super super clean so there's no powder fouling in the barrel whatsoever. There is a lot of "smeary" stuff in it though. Could be copper. Lead looks more rough and builds up around the breech (which there is none). This is pretty smooth looking and evenly distributed. So you might be onto something there. Wouldn't that be ironic if copper is building up, and shooting lead through it cleared it out .

I shoot a heck of a lot of Rainer plated 148 DEWC's through this gun, and I never experience this phenomenon with them. They're very similar bullets.

 

[jag2]

This is totally a guess on my part but I've read about variation in .38s because the case is so large with a small amount of powder. Depending on where that powder is dispersed in the case at the time it is fired can alter how the powder burns, thus changing the velocity. Some have used dryer lint to stuff the case full so the powder stays at the bottom close to the primer. Kind of makes sense. It might be fun to give it a try and see what happens.

 

[Sevens]

I wonder if you tried the same test but with a HUGE interval of time between strings. Literally like an hour. Shoot other handguns and let the revolver sit.

I think we can agree that the buildup of heat is consistent, and is always going to happen. Perhaps that heat and the effect of it only seems to show itself with exactly THAT combination of powder and bullet?

 

[JimmyR]

I'm no reloader, but a revolver guy. I know when I shoot my stainless revolvers, the barrels get really hot, really quickly. While I don't know just how "hot" your loads are (again, not a reloader), would a hot barrel possibly be allowing more gas to escape from around the bullet, thereby reducing your velocity?

 

[TimSr]

I'm going to go with the heat theory. The more the metal expands, the more the bore closes. (I'm not talking about closing in measureable amounts) How much time elapsed between strings? It would also seem reasonable that different platings and coatings would vary the amount of friction, thus heat generated. It would also be reasonable that if the bore gets smaller with heat, that it would affect higher friction bullets more than cast.

 

[stubbicatt]

This is a mystery wrapped in a conundrum!

Perhaps try a cast bullet of the same general design and the same powder charge and see if you get similar results. If you don't, then you know it has something to do with the suspect bullet.

I cannot say what it is of course, but I would guess it is copper stripping from the plated bullets, sort of like you said.

 

[jmorris]

Well, I ran out of ammo. Also, some rounds were getting into the low 600 fps range - and that is my self-imposed minimum velocity for plated (or jacketed) bullets in a revolver. So even if I hadn't ran out of ammo, I was starting to get concerned.

Your loss already started to fall off (spread between the last group and one before it). Certainly it would not fall all the way to zero and at some point you would expect to see pressure signs as of something was plugging the bore.

I suppose you could always just shoot loads that don't act "goofy", I would want to figure out what was going on but that is just me.

 

[Longshot4]

I think it might have to do with the powder. My reason is the burn rate of Nitro 100 is very fast on the chart in fact 4th. fastest. Nitro 100 is also the highest on the Volume Measure Density (VMD) chart. I suggest using one of the more common powders for the 38. Powders such as HP38, Bullseye, W231, 700X... What caused you to use Nitro 100?

 

[243winxb]

Going from lead to plated without a good barrel cleaning may have something to do with it. As the lube is taken out of the barrel by the copper, velocity will get lower.

 

[skizzums]

I would think as the lube wore out the bullet would build MORE pressure. But I think a heating up gun will make your barrel looser and looser until it hits a wall.

 

[745SW]

I'm not familiar with Nitro but my guess is powder charge is way to low. Probably needs to be 4.2 gr. or more.

 

[Doyle]

I can see the pattern. After the range going cold (and therefore letting the barrel cool), the first string was almost at the same velocity as the first string on the original session. This looks to me like the steady decline in velocity is related to the barrel heating up and getting loose.

You can confirm (as someone else suggested) by simply letting the barrel get totally cold then seeing if the next group goes back up again.

 

[g.willikers]

I use the same bullet with a 4" barrel, but with other powders, and haven't run into that result.
So, another vote that the Nitro is the culprit.

 

[Sevens]

I have never used that powder. However, I have used my share of Titegroup.

Titegroup, even at tame .38 Special pressures... is the hottest powder I have ever worked with. It makes cylinders and barrels outrageously hot to the touch. More so than I can easily describe.

Seriously... three cylinders of .38 Spl cooked up with Titegroup and fired all in a row makes a revolver as hot as if I pounded a whole box of ammo through it with another powder.

Titegroup has a high nitroglycerin content. Perhaps "Nitro 100" does also?

 

[Nick_C_S]

I think it might have to do with the powder.

I don't. But that doesn't mean I'm right. I use N100 for my Rainer plated 148's without the same problem. Not to mention a host of lead slugs. That said, I am willing to try another powder. I'll load up a batch with HP-38.

Nitro 100 (NF) is amazing stuff, btw. I'm surprised it isn't more popular. I feel like it's my little secret that no one else knows about. It's super fast so it pops off hard even with low charges. And it's clean. Clean clean clean. As I alluded in post #4, after some 160 rounds or so, the gun is practically clean.

Going from lead to plated without a good barrel cleaning may have something to do with it.

The first test yesterday (and probably on previous days), I started with a clean gun and it happened.

I'm not familiar with Nitro 100 but my guess is powder charge is way to low. Probably needs to be 4.2 gr. or more.

The loading is purposed for ICORE Power Factor. A 158 grainer needs 758 f/s to make PF. With a little extra fudge factor added in for assurance, I design for around 785-ish. I load and shoot a lot of other load recipes (although not with this X-treme bullet - this bullet is new to me) in this velocity range without any similar problems. And my IDPA purposed rounds are even weaker - still, no similar problems.

I'm going to go with the heat theory.

It's possible. But I doubt it. The gun did warm up as I was going through the strings. But these rounds are hardly hot. They're very light action pistol competition target rounds. I've had this gun much much hotter with lots of +P (and then some) rounds, and have never experienced this. The hot gun theory seems unlikely.

Right now, I leaning toward SHR970's copper build up theory. I'll run a bunch of strings with my plated DEWC's to see what happens. And although I don't think it's the Nitro 100, I'll load up some more of these bullets with HP-38 in roughly the same velocity range and see what happens.

I wish Unclenick would chime in. I'd like to hear his opinion.

 

[745SW]

I now realize PF is a element for you yet if nothing else works bumping up the charge to see if it rectifies the issue may help in an informative way.

 

[Nick_C_S]

I just cleaned the gun. And now that the barrel is clean - scrubbed with a solvent laced copper wire brush, followed by wet, then dry patches - when viewed with one of those little light thingies, I can see a slight orange sheen in the grooves. Looks like copper to me. The barrel is otherwise shiny and clean, but there's just a slight orange tint to some of the grooves. Might mean something.