Shot string velocity.

[elkfirst]

I've just added a chronograph to my reloading equipment (Chrony F1). When I go to the range with a clean barrel and fire a couple or three fouling shots, I notice that the velocity steadily increases from the first shot on to about the fourth or so. It then settles somewhat close to my expected velocity based on the charts (usually a bit less). My theory is that this is caused by thermal expansion of the barrel. As it warms up, the bore gets bigger and offers less resistance to the bullet. Is my thinking correct or am I out to lunch? Having this chronograph sure opens up a whole new perspective. /ubbthreads/images/graemlins/crazy.gif

 

[jb1000br]

Well, with chrony's (the brand) always ignore the first shot (this is there advice)

you are seeing the barrel getting fouled. this is common, and you shouldnt worry about it, just remember how many foulers it takes to get to consistant vel's.

not thermal expansion, just dirtying up that clean barrel /ubbthreads/images/graemlins/wink.gif

JB

 

[Mikecr]

My rifles do this as well. It takes all day to work up cold barrel groups and they are twice the MOA of a hot barrel group. Regardless of fouling. I do this for my wood chuck hunting rifles, which are not benchrest guns.
Test it. Shoot/measure 10 shots to foul(mark last velocity), let it cool completely and shoot measure the first few cold barrel shots. I bet they are lower than your 10th shot velocity. And they will likely group differently with a different POI.

 

[wapiti13]

Your clean barrel might not shoot as tight as a fouled barrel, but I would worry about your powder/primer combo. You shouldn't have to warm up your barrel with a good load. After your barrel is fouled, chrono again with a dead cold barrel. If you again gain velocity over several shots, then I would switch powders. That is why Hodgdon brought out extreme powders that don't vary with temperature. RamShot powders are also temperature stable. Many other powders will run 100 fps slower out of a cold barrel. Not good. Are you going to able to warm up your barrel before hunting and needing that first shot? Good luck. /ubbthreads/images/graemlins/blush.gif

 

[Mikecr]

He did mention clean barrel, and I realize fouling affects velocity. But I'm saying that I've seen this plenty in barrels which are fouled already.

A friend I work with is always bringing in ragged hole 3 shot groups from his stock Sendero 7RUM. We're talking .3MOA and below. He's pretty sure I'm pure stupid for spending 5 times as much for custom rifles. He does very basic reloading and shoots off a carpeted 2x4 and sock/sand. Says he can do it all day long. I always wondered how because I couldn't make such a claim off a Farley with perfect 6br ammo.
One day he invites me to his range, and I find that he is not pulling my leg. He can do it. He fires 3 shots in about 15 seconds. Then, lets the barrel cool(1hr) for the next group. He worked up his load this way, and I **** sure wouldn't want to be a dear in his crosshair.
Anyway, I mentioned that BR shooters put down 5 shot groups, and challenged him to try it. When he finally did, sure enough, the last couple moved away from the pack. After another **** hour, I had him shoot 5, with one minute between each. With this he shot 3/4". I told him to move to another target and keep shooting at 1min intervals now that the barrel was warmed up. Doing this his groups reduced some to ~1/2". If his load was worked up for hot barrel shooting, it would probably do better.
He didn't do anything wrong, and didn't need to change anything. It's a excellent hunting rifle, and not a benchrest rifle. Thats what he learned, and I learned alot as well.
If you have a barrel that doesn't shoot differently from cold to hot, it's probably cut rifled, and lapped AFTER contouring. Or it's **** good for some reason unknown to me.

 

[wapiti13]

Mikecr, Very interesting, but in a hunting situation you want the first shot as close to the second as possible. Some powders & primer combinations lend themselves to this, others don't. Especially when taking temperature into account. Does your first shot at 70 degrees match your first shot at 10 degrees? Many hunter/reloaders don't understand the first shot being way off from the second once the barrel/chamber is warm. This was my main point and even more critical at longer ranges.

 

[Mikecr]

I agree with that.

 

[elkfirst]

For sure, my aim is to work up a hunting load. In fact I hunt primarily in Alberta. Deer season starts November 1st, it's not the dead of winter but it ain't that warm either. I will take this advise. Wapiti13, what Hogdon powder do you refer to as "extreme"? how do you know which fall within this category?

 

[Varmint Hunter]

Hodgdon Extreme powders

Take a look here for info on the Hodgdon Extreme powders.

Extreme

 

[Mikecr]

Re: Hodgdon Extreme powders

I wouldn't count on the Hodgdon hype. I don't have a link directly, but did read of some excellent testing that showed Hodgdons is full of hot air. IMR proved less temp sensitive(now owned by Hodgdon), along with the VihtaVuori line.
Think about it. Other than Hodgdon's merchandising, what makes you think they are different in any way? How are they different? Did they sprinkle some magic dust on the Ship loads from Australia(or where ever). What is this stuff?