7 mag reloading conundrum?

I was testing 7 rem Mag loads today and had something weird happen. I was testing 140 gr Berger vlds and working my loads up. I started with the all loads .010 off the lands so oal to ogive was 2.650", all loads are using h1000. First load was 67 gr and velocities were 2715, 2732, 2743. Load 5 69.8 gr velocities 2930, 2930, 2956 and no pressure signs. Load 6 70.5 velocities 2982, 2969, 2969 no pressure signs. So everything is looking good. Test load 7 at 71.2 velocities at 2048, 2020, 2048 what the heck is happening? Test 8 to see it's 72 gr and velocities are 2290 and 2271 I didn't shoot the third because I wanted to make sure I loaded these correctly. Got home pulled the bullet and sure enough 72 grains. So what happened? The only thing I can think of is these 140 gr bullets have such a small bearing surface (.265) that when I seat them to .010 off the lands there isn't much in the neck to allow the pressure to develop properly, maybe 40% of the bearing surface is in the neck, is this logical at all? The 150 swift I measured had a bearing surface of .515 and easily allowed for long seating depth with plenty on bullet being seated. Also I use a redding neck bushing and size all brass so there is .002 (.282 ID) neck tension. Any help on understanding what happened would be appreciated. I'm thinking of going to 160 AB or 168 bergers because I'm thinking this must be a bearing surface issue. Thank you for any help in helping me learn something

 

I'm not sure it was the chrono. I was getting 3009 out of a 270 with 140 gr sst's in their superformace ammo, 2478 out of a 300 savage with 150 ballistic tips and 40.5 gr ogIMR 4895, 813 fps with my 45 acp, 950 fps out of my 10mm glock 29, 1109 with my .22 lr and 2354 fps in a 303 British with factory 180 gr bullets the box said it would get about 2400 fps. All the reading from the chrono seemed good. I'm pretty sure there is a reason why I'm not getting the velocity I should, and not a chrono problem, I'm thinking it has something to do with the bullet not bring seated deep enough but this is just my reasoning and I don't have much experience or knowledge to back this up. I'm hoping somebody with both will be able to shed some light on this. I though the chrono might be bad also , like maybe it means 3290 instead of 2290 but it chronoed the 270 at around 3000 fps just fine. I'm learning that the equipment when it is functioning reliably ( as it seemed to read everything else just fine) is not lying and if it's telling me something is not acting right there is a reason why and not that it is broke.

 

On hodgdons website it list the 140
Noslers partition (note I was shooting Berger vlds but) going I think around 3030 over 70 gr of h1000. I got 2982, 2969, 2969 with 70.5 gr of h1000 with no pressure signs do this is close to book but I was hoping to push em a little harder. What really throws me is why the velocity dropped to 2048 when I went up to 71.2 gr

 

We were facing NE, I started shooting at 12 and these last two groups were shot between 3:20 and 3:45. The sun did come out at around 2:30. The only thing shot between these groups through the chrony was my dads 357 mag and he was testing his HSM Bear loads and those chroned at 1132-1116. Theses two groups shot great but the velocity drop confused me. I will shoot them again the next morning I can get to the range.

 

I will recheck the loads over the chrono as soon as I can. We were having muzzle blast issues earlier so we kept moving the chrono out until everything worked, it ended up about 20-25 feet away from the bench. I was going up in .7 gr loads (1%) at a time. The loads shot great, I dont have the targets in from of me but they were between .6 and .7" at 100 and there was no noticable change in recoil or sound report. And neck tension wasn't an issue, I used the redding competition neck sizer and and rechecked the ID (they measured at .282 before I reloaded the brass. At first I thought I was getting pretty low velocities but after looking at hodgdons website I was just a little slow maybe 50 fps to what there info says for the 140 nosler partitions. Also the chrono was a new competition electronics prochrono digital, so the batteries were freash as well. I was hoping to get 3100-3200 is that realistic in a 7 rem mag with a 24" using 140 berger vlds? I saw that someone had posted they were getting a little over 3200 with an stw and 85 gr of h1000 so maybe Im not not far off of where I should be. I didnt realize that the chrono could throw that far off a number. I got errors earlier and everything else seemed to read clean so I figured it would throw an error if it had a problem reading. The other thing that made me doubt that the chrono could be throwing errors was that it read the strings consistant, I figured if it was reading poorly the numbers would be all over and not close but in the 2000 and 2220 but Im learning so thanks for the info that the chrono can be flakey even if it appears to be functioning correctly.

 

I appreciate the help but you must not have read the tread all the way. I started I about 15 feet and it was reading fine with the .22, pistols and the savage but started shooting the 270 and rem mag and it started throwing errors so I moved it back until it worked. Ended up at about 20-25 feet. It was reading fine and then these last two groups were way low. I believe/understand though that this is still probably a chrono issue but it is most likely from the changing light conditions (brightness or sun moving) and not from muzzleblast. Hopefully sunday I will be out and can recheck these loads.