First shot always slower...
 

I fired 5-shot groups in the 90 degree heat today with 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, and 10.3 grains of Vihtavouri N110. This was my first time to shoot groups through a new CED M2 chronograph and a new AAC 762-SD suppressor mounted on my AR-15 in 300 Fireball. The first shot of each 5-shot group had a substantially lower muzzle velocity. The following example velocities (fps) are listed in the order they were shot:

9.9 grains - 967, 1040, 1049, 1040, 1048
10.0 grains - 999, 1065, 1057, 1068, 1039
10.1 grains - 1004, 1068, 1066, 1070, 1068
10.2 grains - 1019, 1066, 1081, 1084, 1079

I cleaned the barrel and then fired one 9.3 grain fouling round before I started shooting 5-shot groups. I did not clean the barrel during the rest of the session. I normally walk to the target and mark the hole after each shot to let the barrel cool, but today I was having trouble keeping cool so I fired each shot with only about a 30 second pause and only walked to the target after each 5-shot group. That resulted in about 10 minutes between groups during which time I removed the AR from direct sunlight by placing it in the shade of my pickup cab with the windows rolled down.

I was using a Leupold 3.5-10x-50mm VX-III scope and could usually see the bullet holes in the target at 100 yards. The first lower velocity round was hitting the target on the average about 2 inches low. Ignoring the first shot, I was getting from 1.25 to 2.5 inch groups at 100 yards.

I would like to hear the reasoning of others as to why the first round always had a substantially lower velocity. Also, will velocities I am getting now in hot weather be reduced in cooler weather? Should I wait for cooler weather to work up my subsonic loads? Will I have to go through this exercise again when the weather is cooler to come about a subsonic round? I would assume that if the velocity changes with a corresponding change in the barrel harmonics, that the load that showed the best group (10.1 grains) may not be the best load when it is cooler? Any other suggestions?

too_many

I suspect that the first shot being slower might have to do with the powder position in the cartridge. A friend of mine demonstrated this to me the other day with a pistol load. Before each shot he pointed the muzzle down and gently shook the gun and then slowly brought it up into firing position and then touched one off. He repeated this five times and got a very tight group of velocities. Next he pointed the muzzle up and gently shook the gun and then slowly brought it up into firing position and then touched one off. He repeated this five times and also got a very tight group of velocities BUT it was a full 100fps different than the muzzle down results (I do not remember witch gave the fastest results). He then did a five shot string from the draw and the first shot was similar the the "muzzle down" test and the velocities for the next four were between the "muzzle down" and "muzzle up" results.

I suggest that you try a similar test with your loads and see if you are seeing the same phenomena.

As far as the results changing with temp... It depends on how temp sensitive N110 is. I have heard that N100 is not suppose to be too temp sensitive (can anybody else confirm this). You can try an experiment yourself. Put your best load in the freezer overnight ( I would leave it in the mag... I am assuming that you are shooting an AR15) and pack it in an icebox when you go to the range. FIre them quickly before they warm-up too much.

As far as the barrel harmonics that should not change too much with temp but it will change if you change the weight or balance of the barrel by taking a can on or off.

Let us know how your tests go

320pf

Need more testing...
 

I don't think powder position is the problem. The Vihtavouri N110 powder is bulkier than Hodgdon H110 and seems to completely fill the case when seating Sierra 220 MatchKings at a 2.50 inch OAL.

Before I took delivery of the suppressor, I was using Hodgdon H110. I switched to Vihtavouri N110 hoping that it would not be as temperature sensitive because my Hodgdon 9.2 grain subsonic loads at 60 degrees (speed of sound = 1117 fps) had become sonic when the temperature reached 90 degrees (speed of sound = 1149 fps). The speed of sound had increased by 32 fps but my muzzle velocity had increased by 100 fps which I blamed on Hodgdon H110 being very temperature sensitive.

I am going to try a couple of 5-shot groups without the suppressor installed and will report back. I don't know if this is a characteristic of suppressors. If so, I hope that it will not be as pronounced when the temperature gets cooler during hunting season when I will actually be using the weapon.

No Luck...
 

I tried a couple of 5 shot groups without the silencer and experienced the same results with the first shot always being from 40 to 70 fps slower. The first round being slower repeats itself everytime I wait an hour or more to fire the next 5 shot group. All combinations of H110 and N110 powder and Federal 205 and Remington 7 1/2 primers produce the same results. Should I try another powder like H4227?

Since the same thing is happening with two different powders/primers and with and without the silencer installed, I wonder if I am seeing the results of a slight tolerance change in the chamber/throat/barrel due to the heat generated from the first fired round. I am going to try some 240 grain bullets with all combinations of H110/N110 and 205's/7 1/2's.

Any other suggestions?

I have found a sweet spot with 10.1 grains of the N110 powder and 7 1/2 primers that is giving me just less than 1", 5-round groups, if you discount the first round that is always from 1 1/2" to 3" low. The change in impact is what really disturbs me.

I would agree with your statement that the heat is the issue warmer barrel ='s less friction also chamber heat will raise velocity. Trying a temperature insensitive powder like one of hodgdons extreme powders will help. but to some degree the first shot will always be slower unless your bore is oiled and that is not a consistent way to have a first shot.

Ordered ammo to test...
 

I ordered 20 round boxes of 220 grain subsonic ammunition today from both Cor-bon and Reed's Ammunition to test against my reloads.

Theres two things going on here one is as Doc said barrel heating up and allowing faster fps. second is as the powder in the cases reaches ambient temps it will also increase slightly the fps.

How long you leave each round in the chamber will have an effect of fps too.

I don't think that the length of time in the chamber would equate to much as you said you took shots around every 30 seconds or so, this may or may not be enough time for the chamber to warm the powder inside and increase its burn rate. Did you chamber the following round immediately after firing the previous round or used it like a single shot and loaded a round then fired?

I remember a time when I competed at a shooting match where my coach started yelling at me to put rounds down range faster than what I was because the burn rate was being increased due to length of time the case was in the chamber and it was around 30 seconds too, he said I needed to crank one off about ever 10 seconds when the wind was the least amount. Heat will play havoc too as timed shots will allow a more consistent heat soaking for each round.

Come to remember when the wind fell off everyone shot 10 rounds as fast as they could.