Shooting rest while developing loads

[Mikecr]

Boss, I couldn't qualify my observations with any detail. But I have a very good chronograph, calibrated Quickload files, and I see lower 1st shot velocities that vary from barrel to barrel.

I expect that my first shot will be lower in velocity than any followup/heated shot. The bore is cold(er), which slows burn rate. The cold bore is also larger initially, reducing pressure. With the next warm barrel shot, burn rate is getting faster, and the bore is constricting(gradient), raising pressure and velocity. By shot 5 the bore temp should be hot & fairly stable, and temps have sinked throughout the barrel. So the bore is now re-expanding to a stable point, and burn rate stabilizes to provide higher velocity, despite the expanded bore.
Now, even if POI holds through this at close range, it won't at distance. So I focus on the only shot that counts.

Can't think of any reason this would seem uncommon.

 

[Bart B]

Regarding first shot from cold bore then shooting several more, here's two examples of reality.

First, a .308 Win. at 800 yards. Note shots 1 and 2 were the first two from a cold barrel, The other 18 went in centered about 1/4th MOA higher. All shots fired about 20 seconds apart; that barrel was really hot after the last shot.

Second, a .30-.338 starting with a cold bore. Alternating each load and firing a shot about every 25 to 30 seconds. After 30 shots, that barrel was also pretty darned hot. The X-ring's 10 inches in diameter.

Not a significant change in impact going from cold to hot, at least as I see it.

 

[Boss Hoss]

Who built your rifle and what tube is on it?

VERY NICE!

 

[Bart B]

Who built your rifle and what tube is on it? VERY NICE!

Mid Tompkins fit the Kreiger barrels to both the actions; Win. 70 for the .30-.338 and a 4-lug Paramount for the .308. .308's got a 1:13 twist and a 1:12 for the .30-.338. Standard SAAMI spec reamers were used and bolt faces were squared with the receiver tenon threads after facing the receiver fronts square with their tenon threads. I epoxy bedded both using conventional methods with Devcon Plastic Steel; don't believe pillar bedding's worth the trouble. Solid wood stock with cross bolts behind the recoil lug and between the magazine and trigger cutouts on the .30-.338, McMillan synthetic prone one on the .308. All stock screws torqued to 60 inch-pounds.

Both shot slung up in prone with the forend tip and stock toe on a shot bag full of rice. Weaver T20 scope used on both. Could easily hold about an inch on the target. Full length sized cases used with the .308 with Sierra prototype 155-gr. Palma bullets. Sierra 190's from the .30-.338. New, virgin .30-.338 cases used with the Sierra 200's. Mildest large rifle primer made was used; RWS5341.

Three cheers (or fifty in this session) for properly full length sized cases and even the brand new ones in the magnum.

Won my share of long range matches with these rifles.

 

[Bart B]

The cold bore is also larger initially, reducing pressure. With the next warm barrel shot, burn rate is getting faster, and the bore is constricting(gradient), raising pressure and velocity. By shot 5 the bore temp should be hot & fairly stable, and temps have sinked throughout the barrel. So the bore is now re-expanding to a stable point, and burn rate stabilizes to provide higher velocity, despite the expanded bore.

I don't understand this at all.

All the round steel parts with holes in them I've measured with precision pin gages get larger hole diameters when they get warm. They get larger as heat goes up. They only get smaller as they cool down.

What physics and properties with steel are there that causes a rifle barrel bore (as you state above) to get smaller as it first heats up then get larger with more heating?

 

[Boss Hoss]

Bart---sounds like you have a couple of Hummers There!!! Take care of them!!!!

 

[Mikecr]

Bart & BOSS, I was not speaking in terms of POI, but velocity from cold to hot. So I believe we're on different trails. I should have stayed with POI for this discussion.

Bart heated metal expands, while cooler metal does not. Right?
When You fire into a cold barrel, the metal nearest the bore becomes hotter than surrounding metal. This hotter metal will expand in the direction of least resistance, because it can. That is inward, until the rest of the barrel heats up to allow outward expansion.
Drill a hole in a plate, heat the edges of the hole(rather than the entire plate), and watch the hole constrict -initially.
You can see it with pin gauges from shot to shot. You can see it with a chronograph from shot to shot.

But none of this directly correlates to POI, as ladder tests often show. Too many other barrel stresses + load timing to predict POI from cold to hot & from one barrel to another.
I've been lucky with a few barrels that hold cold to hot POI, but most of mine haven't.
However, they can still be consistent and fine for single shots, demanded with hunting.
So it's my contention that group shooting, however impressive to some, doesn't really relate to our needs here.
In fact, it can be counterproductive.

 

[Boss Hoss]

Mike---I am talking about Cold bore. I beg to differ because if you have a cold shot that has the types of velocity variation you are advocating here then the POI will be very significant at 600 and 1K so as we can see group size does matter. Cold shots are something that is done when hunting everyone knows that. My point is that the cold shot is not necessarily going to be materially different than the next one. Will agree that if you are at 0 degrees and shoot 5 rounds quickly there could be a difference because I have never done that.

The other part of the equation that you did not address is how long you leave a round in the chamber before firing because heat soaking a round in a hot chamber or warm chamber for that matter will change the pressure curve. When shooting for velocity and group I NEVER chamber the round until my sight picture and condition is perfect and the trigger is ready to be pulled. If you chamber a round and wait 30 seconds or a minute maybe more the heat is transferred to a greater extent and you have inconsistency.

That helps me win in competition and helps me extract the most accuracy potential from all of my rifles. First cold bore shot is the most important which is something that only a fool would argue. My point is that most of and ALL of my primary hunting rifles do not vary in velocity enough to be statistically important.
This is one of my primary rifles along with the rest---I am pretty serious about my accuracy and my Smith is one of the best there is in building accurate long range platforms so my expectations are pretty high when it comes to first shot capability.

If your program works for you then great---mine works for me.

 

[Mikecr]

Boss, we've missed each other in context.
I wasn't talking about hot or cold powder, or outside temperatures, or fouling, or ES.
I was referring to velocity climb from 1st shot of the day(cold) to any that would follow to make a group. A gunsmith cannot influence this, and it's not a problem for any of us to account for.
But it is important to do so.
So far I've used barrels from Lija, Pacnor, Border, Snyder, Wilson, Krieger, and a factory Browning. The cut rifled hold transient POI best, but every barrel I've seen to date climbs in velocity from the first cold shot to warm to hot. They all do it, there is no way around it. 'Sighters' in competition get the bore fresh fouled and up to stable temps, -before shooting a string for record.
As you seem well aware, this is not what hunters do.

Nice gun, looks Speedy built.
But anyway, this is a hijack now

 

[Clark]

I try to chase down every post Bart makes

Bart Bobbitt May 18 1994

Tests have shown that muzzle velocity increases about 1.5 fps per shot in
a .308 Win. and about 2 fps per shot in 30 caliber magnums. Which means
you need to come down on your sights about 1/4th MOA every 5 or so shots.
Many times, I have had to stop shooting at 1000 yards for any one of
several reasons but had a round in the chamber. Previous tests showed that
I need to come down 1/4th MOA on my sights for each 20 to 25 seconds of time
that round `cooked' in the chamber. More than once, I've layed there with
my eye on my timer and comming down a click, then another, another...even
a total of two MOA on occasion, then finally shooting the shot, calling it
good and it nails the X-ring. After that shot, I move the sights back up
to the normal setting, fire, and it goes to the same place.
When coaching a shooter using a scoped rifle at long range, you can just
tell him to break the shot at 6-o'clock in the X, 10, 9 or 8 ring depending
on how long the rounds been the chamber. This works very well.
This practice is not a trick; just simple physics. It's best to test your
rifle/ammo combination to find out how much bullet impact climbs with
a hot round. It'll well be worth it.
BB

 

[kcebcj]

Boss....I like the rear bag in the photo you posted. Who makes it?

 

[ReachnOut]

BOSS .... Not to get off topic, but that spiderweb on your barrel looks very familiar ..... Speedy?

 

[Boss Hoss]

The bag is Edgewood and Speedy builds all of my competition and SS Sporters. I have quite a few and FWIW I did the spider webbing and the marking on that rifle in Speedos shop of course.

 

[ReachnOut]

The bag is Edgewood and Speedy builds all of my competition and SS Sporters. I have quite a few and FWIW I did the spider webbing and the marking on that rifle in Speedos shop of course.

Great to run across another fan of "Speedy" rifles. At last count there were 9 in the safe. Glad to know there's someone good at the webbing and marking. If you get too much spare time, I've got about 4 rebarrels coming up!