5;56 velocity change and don't know why

[Tucker]

Bullet weight: 55 gr
Powder: 23.0 grains H4895
Brass: Mixed headstamp
Rifle: Standard AR W/ 16 barrel (twist unimportant)

I've used the same load for 20 years. It had a 2900 fps give or take. Since I used range pick up, that caused some deviation. I put a new optic on it and wanted to zero it at 100 yds instead of 50. Took the chrono out and ran some so I could get a reading to make a ballistic drop chart. Avg velocity was 2550 fps on 20 rounds. (two sets of 10). Came home and picked out 10 pieces of brass with same headstamp. Measured the load by hand then back to the range. I called a friend to meet me there with his chrono. We set them up end to end. Same results. 2550 avg.

I cannot figure this out. Powder was stored on a shelf in the basement. 8 lb jug.

What's your thoughts?

 

[spife7980]

So the new random head stamp you chose is giving you different results than the old random head stamp?
Well, duh.

Measure their internal volume between the two head stamps and you’ll find one is larger than the other, likely has a thinner case head as well.

Sort your brass, use the same type consistently and I bet you’ll have more consistent results. Similarly to how the new head stamp shoots consistently slower.

 

[nn8734]

Bullet weight: 55 gr
Powder: 23.0 grains H4895
Brass: Mixed headstamp
Rifle: Standard AR W/ 16 barrel (twist unimportant)

I've used the same load for 20 years. It had a 2900 fps give or take. Since I used range pick up, that caused some deviation. I put a new optic on it and wanted to zero it at 100 yds instead of 50. Took the chrono out and ran some so I could get a reading to make a ballistic drop chart. Avg velocity was 2550 fps on 20 rounds. (two sets of 10). Came home and picked out 10 pieces of brass with same headstamp. Measured the load by hand then back to the range. I called a friend to meet me there with his chrono. We set them up end to end. Same results. 2550 avg.

I cannot figure this out. Powder was stored on a shelf in the basement. 8 lb jug.

What's your thoughts?

My thoughts?

Stop using random, stepped-on range brass.

Purchase large quantities of LC once-fired from whereever you can find it. The stuff can usually be had in bulk for fairly cheap and is perfect for any use, except maybe where extreme precision is required (you'd step up to premium brass in those cases).

I use it in all my carbines and Mk12 and does everything I need it to from a consistency perspective and is easy to work with.

 

[Tucker]

So the new random head stamp you chose is giving you different results than the old random head stamp?
Well, duh.

Measure their internal velocity between the two head stamps and you’ll find one is larger than the other, likely has a thinner case head as well.

Sort your brass, use the same type consistently and I bet you’ll have more consistent results. Similarly to how the new head stamp shoots consistently slower.

Came home and picked out 10 pieces of brass with same headstamp. Measured the load by hand then back to the range. I called a friend to meet me there with his chrono. We set them up end to end. Same results. 2550 avg.

 

[Tucker]

My thoughts?

Stop using random, stepped-on range brass.

Purchase large quantities of LC once-fired from whereever you can find it. The stuff can usually be had in bulk for fairly cheap and is perfect for any use, except maybe where extreme precision is required (you'd step up to premium brass in those cases).

I use it in all my carbines and Mk12 and does everything I need it to from a consistency perspective and is easy to work with.

Would that account for a 300 fps drop?

This is my 3 guns load. Doesn't need to be super accurate.

 

[nn8734]

Would that account for a 300 fps drop?

This is my 3 guns load. Doesn't need to be super accurate.

Probably - do testing to rule it in or out based on what @spife7980 said....Mixing brass head stamps is begging for shit like that to happen. Eliminate that as a variable so you don't have to worry about it.

 

[Doom]

Something is wrong with your load and velocities. 23 gr of H4895 is ridiculously low. Your issue between 2900 and 2500 is NOT likely to be case related.

H4895 is not really a good powder for this weight bullet. Looking into Gordon's Reloading Tool 2500 to 2600 fps is about what you would expect from 23 grains and a 55 gr bullet.

 

[sinister]

Would that account for a 300 fps drop?

How many rounds down the barrel? As your throat advances you'll get some blow-by.

How old is the powder? All nitrocellulose powders start to decompose the moment they're manufactured. Diphenylamine is usually added as a preservative but it can't stop oxidation.

 

[spife7980]

Came home and picked out 10 pieces of brass with same headstamp. Measured the load by hand then back to the range. I called a friend to meet me there with his chrono. We set them up end to end. Same results. 2550 avg.

Yes, you said that.
But you also phrased it so that this new brass that gave you a consistent 2550 on multiple weekends is different than the brass that was giving you 2900 for 20 years. So with the evidence you have presented, I deduce that the differences are between the brass you are using.

To get anymore specific in guesses will require you to provide much more specific data.

 

[RTH1800]

I do not think it’s brass. Change is too great.
I’m suspecting shot out barrel with powder issue a possibility.

 

[johnnyvw]

How long has that 8 lb bottle been "sitting on the shelf in the basement" since it was opened?

 

[RTH1800]

Is is possible that the initial velocity readings were incorrect or mis remembered?

 

[Tucker]

I do not think it’s brass. Change is too great.
I’m suspecting shot out barrel with powder issue a possibility.

Other's have made that same deduction.. It is the logical reason when it was fine before. The gun has several thousand rounds down the tube.

 

[Tucker]

How long has that 8 lb bottle been "sitting on the shelf in the basement" since it was opened?

About 3 years. Had surgery and was down for over a year. Slow recovery after that.

 

[ShtrRdy]

Is the area the powder has been stored on the humid side?

 

[Near miss]

What I am feeling out is that you messed up the correct powder charge in your head.
All the signs came to with the "This is an old load of mine" line.

Not the first one and not the last time.

Go check your notes if there is some spot where the charge has just spontaneously changed.

 

[johnnyvw]

Hmmm...along those lines, have you been checking the calibration of your scale all along? Might be an issue with the scale being off