Home Forums Hunting & Shooting Ask The Gunwriters Chronograph and barrel heating

Has anyone else experienced this? I ran nine rounds through my chronograph today without stopping pretty much to just use up these last nine rounds,,, but I wanted to get a velocity ,,,,the first three shots averaged about 2660 OK that's good I kept firing and each shot after that, the velocity slowly went down. Down to 2505 now this is a very pencil thin lightweight barrel and the barrel got pretty hot so my question is as the barrel gets hot should the velocity go down like that? I've never seen this before, but I never chronograph nine in a row in rapid order .

I would have expected it to go up slightly as the chamber heated up.

Thermal expansion.

Along with thermal expansion comes gas leakage past bullet.

Just what I need....another rabbit to go down. Potential conclusion I hadn't considered.

Which chronograph?

John, it's a shooting crony

Hmm. Due to quite a bit of experience with Chronys, I suspect part the "problem" might be with the chronograph.

Mule Deer;
Good morning John, I trust you and Eileen are well and that the Montana Banana Belt is living up to it's name.

Our spring has been a wee bit later than normal, so the Saskatoon bushes and Arrowroot Balsam are just now blooming in earnest, but it froze again last night so we're not chomping at the bit too hard to get gardening.

While I'm sure you remember helping me out with a similar issue a few years back, I'll relate it again in the event that someone out there can glean some wisdom from it.

My older Chrony would read consistently with the .22 ammo that I used to test it for function, but as velocity increased its error got worse and worse. It was reading too fast.

As I recall, at first you'd suggested I find a buddy with a chronograph to double check and sure enough it was no longer reading correctly.

If memory serves we shot .22 rimfire, .17 HR, 6.5x55 and .308 Norma across it.

Also the crazy thing was it was fairly consistent in it's incorrect readings, that is to say the velocities within that grouping were fairly close, but way too fast.

Lastly when we discussed different brands that I could find up here, from them you said you'd had good success with a Pro Chrono, which I subsequently bought.

As mentioned, I absolutely love it and have had zero issues with it. It is much easier to set up and use in different light conditions.

Thanks again for the help back then John and all the best to you both as we head into spring.

Dwayne

A lot of Chrony users don't exactly cry when a stray bullet lands dead center.

I tried the original green version and it registered magnum velocities with standard cartridges. Always about +200fps over book. Sold it to a bud who wanted it and he shot it.

Stayed with Oehler after that as velocities were industry respected as both accurate and repeatable. A rather handy feature I thaught.

Its always easiest to blame the Chrony. I know you have done the same test as me, a chrony in front and behind a Oehler 35P and found not much of a difference.




So he has a pencil barrel that he got hot. Velocity goes down so blame the chrony when there can actually be more to it. Thermal expansion is a real occurrence in rifle barrels and directly to rate of fire. Depending on the thickness i.e pencil to heavy varmint the effects show up or are negligible.

Anyway heres the formula, and test rifle was 22LR.

The question whether temperature changes have a significant impact on the hitting point should be answered. From a physical point of view, the rifle barrel is a heavy walled tube (in the case of the tested rifle: inner diameter 5.6 mm and outer diameter 22 mm). The changes in the barrel temperature cause the barrel to change its dimensions in the process of thermal expansion of solids. How important is this change? In the case considered here, the dimensions of internal and external diameter of the barrel change according to the formula for linear expansion of solids, i.e.

Φ = Φ0(1 + α ⋅ ΔT)

where Φ0 and Φ are the dimensions of the considered barrel diameter before and after shooting, respectively, ΔT is the increase in barrel temperature and α is the thermal expansion coefficient of steel equal to 1.2·10−5 1/.

The inner diameter of the barrel, with an increase in temperature of 5°C, would change from nominal 5.6 mm to 5.60034 mm, i.e. 0.006%.


Getting people to think outside the box for explanations can be tough.

Swifty,

Go back and reread my answer: I did NOT blame the Chrony. Instead I suggested it could be a possible cause, having seen wonky results from them many times--which is why I asked what chronograph was used. (And from the comments on Chronys since mine, quite a few other people have too.)

But I will mention something else: When working up handloads for "colony" varmint rifles I generally end the tests by shooting a 10-shot string with the selected load as fast as possible, to find out if the barrel "walks" as it heats up--and usually chronograph the load at the same time.

A surprising number of barrels do shift POI, even heavy barrels--but I've also owned (and own) many sporter-weight varmint rifles, some with very slim barrels. Cannot recall ever seeing velocity drop during those 10-shot strings. As somebody else already suggested, velocity tends to increase as a barrel heats up, even with more temperature-resistant powders.

John

Hi Duane,

Actually, our part of the Montana "banana belt" has had a similar spring. In fact, today's weather forecast is predicting possibly heavy snow on Monday! In fact snow-says have interfered with our usual level of spring rodent-shooting, partly due to muddy farm/ranch roads, and partly due to Richardson's ground squirrels preferring sunny days, which not only feel nicer but result in "warning shadows" of approaching birds of prey. Hope to do a little more shooting before Monday!

Good hunting,
John

John,

My first Chrony, purchased for $50 from L.L. Bean of all places in the early 1990s, did die from a stray bullet. But I learned another thing about Chronys with my second one, which worked pretty well until the readings went wonky a few years later. I sent it to the factory, and when they shipped it back they reported the problem had been dust on the sensors--and dust isn't unusual in this part of Montana, especially in summer and fall. So I placed a piece of transparent tape over the slots above the sensors, something I've also done with other light-screen chronographs since then.

The readings didn't change when testing the same ammunition until a few years later, when they slowly started rising--something I didn't figure out for a while until retesting some of the .22 Long Rifle "control" ammo that I'd used since purchasing my first chronograph in 1979.

About that time Oehler start offering their 35P again, so I bought one, which I still occasionally use partly to check out the accuracy of other chronographs. One of the results was recommending Competition Electronics Pro Chronos to shooters looking for an inexpensive chronograph that actually works consistently.

John

What are users considered opinions on the PACT Model 1?

Sho got dat right!!!

One other range member prevented me from shooting my Chrony when it pushed me to my limit.

I've seen two Shooting Chronys that would 'stack' the differences for each shot and add each difference to the displayed value. The more you shot, the wonkier the results.

Pretty hard to beat the Competition Electronics Pro Chrono for dependability and price. Mine has been trouble free for many years.

Good shootin' -Al

It's been many years since my Engineering Materials Science courses, but your formula looks to be correct and I'll assume the thermal expansion coefficient used is correct as well.

To put some perspective on your end result, a temperature change from 0 degrees C to 100 degrees C would result in a 0.00027 inch bore diameter change. I find it difficult to believe that enough expansion is occuring to exceed the obturation ability of the bullet, allowing blow by and creating the velocity drop being experienced by the O.P.
Something else is happening IMO.

not really relevant here but talking about chronos doing wonky things..
I discovered some time ago with the pro crony pal and digital models when running them off of 9 volt battery when the battery starts getting low your numbers will go up...
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I have a plug-in adapter I use on my private range for it's plugged into 110 outlet to alleviate that issue..

Yeah, you have to either change batteries (I always have extras in my range bag) or use a system like yours.

But one of my other Shooting Chrony experiences was on a local semi-public range. The only other other guy shooting that morning had a Chrony, and it was registering velocities 800 fps below "normal" with his pretty standard .30-06 load. We started talking and I suggest changing the battery. He didn't have an extra, but I did: Same result....