Model 10 - strange chrono results, first round always slow?

[Fishstyx]

Hi all. New to the forum and relatively new to reloading. I have my fathers ~1962 Model 10 which I've been shooting on an off for years and collecting cases. I recently picked up a few hundred Ranier plated 148 grain DEWC and 125 grain plated HP.

I loaded up several five-round groups with increasing charges of Titegroup and went to the range to measure through my ProChrono. I took along some factory 158gr LRN and 130gr FMJ to baseline. I shot 12 groups and with the exception of the first group, all the first shots were slower, often much slower than the rest.

I'm wondering why this is. It seems too consistent to be by chance. Is this a typical phenomenon? The barrel is surely much cooler for that first shot. Maybe there's an error in the chrono. FWIW, these Ranier plated bullets are right at the verge of being oversize, though they are within spec.

My data:
1st group was handload 158gn Winchester JHP-notched bullets with 3.8gn Titegroup. The results were tight, 713FPS average, SD of 13.

2nd and 3rd were factory ammo:
130gn FMJ, Avg: 807, First: 750, SD:33
158gn LRN, Avg: 714, First: 688, SD:20

Rest were handloads with the Ranier plated bullets.
148gn DEWC: (5 shots each group)
Charge: 3.0gn, Avg:641, First:567, SD:47
Charge: 3.2gn, Avg:667, First:650, SD:18
Charge: 3.4gn, Avg:754, First:703, SD:33
Charge: 3.6gn, Avg:769, First:741, SD:25

125gn plated HP: (5 shots each group)
Charge: 4.1gn, Avg:705, First:530, SD:117
Charge: 4.2gn, Avg:738, First:648, SD:52
Charge: 4.3gn, Avg:759, First:642, SD:71
Charge: 4.4gn, Avg:774, First:681, SD:58

If I delete the 1st shot from each group the SD for each is less than 20. Clearly the plated hollow points are the worst with this slow 1st round effect.

I'm throwing powder with a Hornady auto-charge.

Does this make sense?

 

[RWJ]

Powder positioning

I've heard of the position of the powder in the case moves to the rear after firing the first shot. A former colleague had this happen when he went to shoot IPSC(?), and they told him his loads were under the power factor. Elevating the muzzle before each shot, and then chronographing, made the difference, and he was allowed to compete. I've never bothered to check my own loads, as group size was the only thing I was interested in PPC.

RWJ

 

[gboling]

I would suspect the Chrono. However I have nothing to substantiate that. I would try the same test with another gun, preferably with another caliber. If this happens with different guns and calibers I would suspect the chrono. What make and model is the chrono?

 

[gboling]

What RWJ said could be the cause. Try pointing the gun up before each shot to see if the first shot becomes consistent with the others.

 

[H Richard]

The only way to get consistent chrono results is to hand weigh each charge. Charges thrown by a auto charge may not be consistent. Especially just after changing the load.

 

[Fishstyx]

Thanks for the replies.

The Hornady auto-charge (the robo trickler on a scale as opposed to a volume thrower) weighs each charge so the powder weight should be correct.

As for the chrono, it's a ProChrono Digital. The conditions were ideal (not a single missed bullet). I have data from the same day for a 17HMR and AR-15 and didn't have the slow 1st bullet effect.

I'm partial to the powder in the case explanation. Titegroup flakes stick to everything so I can imagine the inside wall of the case coated with flakes of powder that doesn't burn uniformly. The first shot may shake the powder loose in the other rounds so it burns better. Look at the two factory ammo strings. The first shots were slower, but just barely with a tight SD.

I think the bottom line is that I need to try some different powder (Unique if I can find it). I'm going to load up another batch and give it a go this week to see if I get the same behavior. I'll also trying knocking the rounds like a pack of cigarettes before I load.

Do the velocities look right? I thought they were sort of slow in general.

Lastly, I really need a 357.

 

[Alk8944]

Fishstyx,

How you present the revolver for each shot can make a difference. If your habit is to load the gun muzzle down and then simply raise it to level for the first shot then what RWJ suggests can well be the issue. Most people would raise the muzzle when cocking for the next shot which would then tend to settle the charge to the rear of the case.

During the past nearly 25 years I have chronographed well over 100,000 rounds, mostly from revolvers. Consistency of powder position makes a big difference in consistency of results. Average velocity with the charge deliberately seated to the read of the case as compared to deliberately moved to the front will generally result in average velocity at least 100 FPS higher, lower ES and lower SD.

 

[jmmitc06]

Yes the SDs on the factory groups are smaller but what would matter here to confirm the presence of a 'first bullet effect' is the distance from the mean of the first bullet in terms of SDs, the so-called Z-score or standard score.
Assuming they are normally distributed, which may or may not be the case, the factory ammo samples have Z's of 1.72 and 1.3 (for 2nd and 3rd factory ammo data sets). Your reloads have Z's of 1.57, .94, 1.54 and 1.12 for 148gn DEWC samples respectively and 1.49, 1.73, 1.64 and 1.60 for 125gn plated.

Based on this I would conclude that the consistency of the readings for the reloads and the factory ammo do not differ considerably, it is unlikely then that the fact your ammo is reloaded is a cause of this effect. Also, perhaps the JHP factory would have a similar z-score for the first bullet as these other samples even if the actual difference is very small because the SD was small.

Second, a scientist (like myself) would conclude that there is no first bullet effect because the first bullets do not differ by two or more SDs, it's not a statistically significant result. Of course there may be such an effect but this data does not demonstrate it very well (small sample size could effect this). However, let us assume there is such an effect for the time being.

Third using the T-test for two independent samples, the means of the reloads were compared to the factory and I found that they were only significant at 90% confidence and not at 95%, so they really don't differ all that much. ALso between your reloads there is no significant difference in means, indicating that the type of bullet used is not a major contributer (plated may be worse, but not by a signficant margain). From a stats perspective, all of your reloads are from the same population.

With all of this in mind, I think the following could help explain and prove the existence of this effect. All of your samples seemed to demonstrate this behavior so it must be a experimental setup problem or something done to all the samples, the first thing that comes to mind is the chrono. Yes the 17HMR and AR-15 appeared normal but at hihger velocities the device could have tighter tolerances or again from a z-score perspective could have had the same problem. Without knowing the tolerances of your chrono, it's hard to be certain this isn't a factor, I would try to replicate this behavior on a different chrono to rule this possibility out. Second, as for the sticky powder hypothesis shoot one round at a time with one round in the cylinder at a time. If your hypothesis is correct you should see this behavior every time. But if the factory ammo was doing this too, idk how likely this is, but definitely a possibility. Lastly, the variance in the ammo can still be due to the charge weight, even the auto charge has tolerances. While I find it unlikely that every first round would be low, I think you have to weigh the charge to prove this phenomenon occurs and is not due to inconsistent charging. Also, if as a previous poster said, the machine has trouble switching charge mass accurately, that could be why the first round is always off, try shooting them in a different order than they were reloaded (if you don't already). Are you using a rest? If not, I would try that too.

I think this would be a very interesting experiment to prove this occurs and to explain why.

Sorry for the length of this.

 

[donadler]

In SASS competition, if a competitor has to have his bullets chronographed, the procedure is to elevate the gun prior to each shot, IIR, so that the powder is distributed to the rear for each shot, such as suggested by a post above, so I am also of the humble opinion that it is the powder position that makes the first shot, with the powder closer to the bullet than the primer, slower than the rest. And oddly enough, I almost understand all of Jimmitc06's post!

 

[Fishstyx]

Jmmitc06 that's a lot to chew on.

I appreciate the analysis. I agree, i need more data, variable control and secondary validation. I loaded up another batch and tomorrow I'll see if I can find some alternate powder locally. This time I checked the charges thrown by the auto-charge on a separate beam scale by balancing the first charge and making sure subsequent of the same weight balanced identical to the first.

The rounds were separated into little plastic snack bags so they were fired at random compared to load order.

I'd love to put another chrono behind the one I have to compare, but that's not possible right now. I also need another 38. My wife already realizes this is going to be another expensive Rabbit hole.

I'll post some pics of the next exercise

 

[scooter123]

I suspect that you'll find doing the "barrel tip" will provide a solution even if it does sound a bit like a dance. As for an alternate powder I'm not really a fan of Unique due to it's poor metering qualities and tendency to shoot really dirty at low pressures. Since you have a power trickler you probably don't care about the poor metering but it's still rather filthy shooting at low pressures. When you are out looking around I would suggest that you look for some IMR SR-7625, it meters extremely well and is quite clean shooting at reduced pressures.

My Lee manual doesn't list any data for the 7625 with a lead wadcutter but does list a 3.7 grains starting load for a 146 grain jacketed bullet at 640 fps and 4.2 grains max at 810 fps. With the weights being so close you'll probably be safe using that 146 grain data. Obviously you would need to start light and work up to a useful load to confirm this.

 

[Jim Watson]

Advertising to the contrary, Titegroup is quite position sensitive.
As said, you will get more uniform velocity by elevating the muzzle for each shot.

I think you are accentuating the effect with those plated bullets, especially the 125s. Lots of airspace in the case, high friction copper on steel down the barrel.

 

[sctman800]

I have to agree with the powder position in the case especially since Titegroup fills such a small volume of the case. I have never checked but it is my understanding that most factory rounds pretty much fill the case by volume. This would negate the problem of case position of the powder. Jim.

 

[Fishstyx]

Ok, I picked up some IMR SR-4756 which required roughly twice the powder weight as Titegroup. This was also the only 38 Special compatible powder they had in stock.

I also picked up some Hornady 125gn and 158gn JHP. I'll make these up tonight and should be able to shoot them tomorrow if the weather cooperates.

 

[Fishstyx]

I made it to the range today and fired 86 rounds in 18 strings according to a plan. The details are too much to try to squeeze into this post so here's a link to a PDF of the results:

Results

The bottom line is that powder position is critical. Despite a small handful of outliers, every time I had a slow reading I realized I forgot to consciously tap the powder to the back of the case. When I remove those from the average, I get numbers very close to the Lyman book (which tested with the exact same bullet/powder combo). My Ranier plated bullet results were a little slower than last time. The only difference there was I resized the loaded cases.

For my last string I loaded 6 of the Winchester 158g JHP in front of 3.8g of Titegroup. I shot three after tapping the power to the front and three tapping the powder to the rear (muzzle up). Those results were interesting.

Lastly, despite only having one bullet fly over 900fps, I did see signs of over-pressure (flattened primers) on some of the max load strings. The flat primers coincided with the fastest rounds.

 

[Fishstyx]

Here's a pic of one of the groups with some showing signs of high pressure. Specifically the one at 10 O'clock

 

[Peak53]

Thanks for the follow up range report. This provides some more evidence that powder position, especially in smaller volumes is important. I used to be in the habit of starting with a muzzle up attitude, but that somehow slipped away. I'll pay more attention in the future.