Bullet Velocity vs barrel clean or dirty

slimjim

I have been trying to determine my bullet velocity for my 270 Win. I'm loading Barnes copper banded 110gr TTSX in front of 60.5 grains of IMR 4350. I don't have a chronograph yet so have been doing velocity estimation by shooting at 100 yards then deriving velocity from bullet drop at a longer range like 250 or 325 depending on the range I'm at. My first derived velocity was 3450 fps which is within a few feet of load manual data. Then one day while at the range, a shooter next to me let me use his chronograph. The first shot was 3410 fps and the next 3 were very close together at 3700 fps (I'm not showing signs of overpressure). The only difference between these shots were the first one was through a clean, unfouled barrel. I was skeptical of the higher velocities until I shot a few weeks later at 410 yards and 550 yards and my impacts were high. If I went back into the ballistic calculator with 3700 fps, the differences at both ranges were accounted for. Then last week I had the chance to do a derived velocity check at 100 and 325 yards and the group average was 3375. This time, however, the barrel hadn't been cleaned in 60 some rounds. I have not been getting much fouling at all with the Barnes bullets and a cleaning after this shooting session still showed little fouling (longest I have gone so far with this rifle). Thanks for getting through the background.

Has anyone experienced velocity changes based on a clean or fouled barrel? Other suggestions besides try another chronograph? I hope to be going back to the 325 yard range next week to repeat with a clean barrel.

Bushmaster

I do have a chronograph and see little difference in fps from a clean dry barrel or a several shot fouled barrel. Maybe +/- 50 to 100 fps difference. I do, however, notice that some of my rifles shoot better clean and some don't. Of course I don't clean after every shot so this is a flawed statement too.

chambered221

Did you account for any head or tail winds that could affect POI ???
I find the 250-300fps jump to be odd regardless of a clean/dirty barrel!!!

Pumpkinslinger

How many rounds are you firing at each distance? How are you measuring the changes in drop? If I understand correctly I don't think this method could give you much more that a ballpark figure because there are too many variables. Chronographs aren't all that expensive... Wink

stovepipe

I've read some barrles can "shoot into a sweet spot" improving or affecting accruacy and consistency. (kinda off topic, I know....)

It made sense to me. Just as some barrels/guns like a particular load over others.

Anyone else read this or something like it?

PaulS

All gun barrels undergo a resonant vibration when shooting. This means the barrel is whipping - sometimes in a rotary motion - on each shot. If the bullet exits the barrel closer to the Peak (high or low and right or left) then small variations don't change your point of impact. If they exit in the middle of the whip - when the barrel is moving faster then even small variations can change the point of impact a lot.

When I work up a load I find the most consistent round over a few months of testing and tuning the loads. Then I get the chronograph out and check the velocity so I can work up ballistics tables. That is how I get sub MOA groups (sometimes sub-caliber) consistently.

chambered221

Good point Paul!!! I believe we had a discussion on this a long time ago???

stovepipe

Those are really cool fine points and I've only read about them....

slimjim

Pumpkinslinger wrote:

How many rounds are you firing at each distance? How are you measuring the changes in drop? If I understand correctly I don't think this method could give you much more that a ballpark figure because there are too many variables.

Sight your rifle in for 100 yards and verify with a 3-shot group. Reposition to 325 yards (max distance at the range I shot) and shoot at least 3 shots and measure bullet drop. Bullet drop was -10.9 inches. Go to your ballistic program and change velocity to match bullet drop. If you BC is good, there is no reason that this should not be an effective way to measure velocity.

Wind only changes left-right impact with any significance which doesn't affect a velocity measurement. The rifle/load is shooting about a 1/2 MOA so there isn't much variation in impact. Last 3-shot group at 325 yards was an inch. One of the groups had a vertical variation of only 0.25 inches which should be an indication of consistent velocity. I'm suspect of the chronograph I used. I definitely need to collect more data at longer distances.

Chronographs have gotten fairly inexpensive though I'm not sure I have room for one more thing in my garage.

fnuser

Pumpkins right, 325 fps difference isn't where it is. I've chrony'd new barrels that have never been fired from hart, and virgin clean to dirty you could account for maybe 100 but even that is stretching. if you like doing math recalculate the difference in velocity as it is related to pressure and see how much pressure would be required to gain that 325 f.p.s. As you probably know since this is a diminishing return it would take quite a bit.

Donut Slayer

PaulS wrote:

All gun barrels undergo a resonant vibration when shooting. This means the barrel is whipping - sometimes in a rotary motion - on each shot. If the bullet exits the barrel closer to the Peak (high or low and right or left) then small variations don't change your point of impact. If they exit in the middle of the whip - when the barrel is moving faster then even small variations can change the point of impact a lot.

When I work up a load I find the most consistent round over a few months of testing and tuning the loads. Then I get the chronograph out and check the velocity so I can work up ballistics tables. That is how I get sub MOA groups (sometimes sub-caliber) consistently.

Thats how i do it too. Work for accuracy first then get the tables for the velocity later.

chambered221

Been thinking about them 3700fps readings!!!

It is entirely possible that a shadow has caused the second pickup to trip early giving you false readings!!!
One thing that I learned early on with chronographs is that any time you get a reading that don’t seem right or no reading at all it can usually be tracked down to a lighting issue. That’s one of the reasons light kits are available for the better units.
If the sun was low and angling in from the back it could cause a shadow that trips the unit before the bullet actually passes over. I’ve personally had this happen while shooting a 9mm.
wtf

I was getting velocities just over 1200fps with a 147gr bullet. It wasn’t until a cloud covered the sun and velocity showed 850fps that I realized what was going on. I moved the unit to another shooting bench and proceeded with my testing.

When using a chrono it’s best to obtain data over multiple shot strings and even multiple days to get a true average.

SingleShotLover

It stands to reason that shots from a fouled barrel might be slightly higher than from clean barrels simply because of the fouling causing slightly higher pressure...but the differences should only be in the statistical range and certainly not around 300 fps difference. I agree with 221 that you need a wider range of results before relying on your original readings.

Dawgdad

chambered221 wrote:

Good point Paul!!! I believe we had a discussion on this a long time ago???

still fresh in my mind... Shocked

slimjim

I went out yesterday and refired at 325 yards with a clean barrel. There was no noticeable change in impact point, thus no difference in velocity. The new groups printed exactly over the old. Thanks for the feedback.