CMV
Member
Why would I get signs of overpressure with 55 gr V-max that I do not get with 55 gr Hornady FMJBT?
I've run LC brass trimmed to 1.751-1.752, CCI #41, H335, OAL 2.220 & Hornady 55 gr FMJBT up to 26.3gr with no issues. Out of a 20" 1:9 AR they go right around 3300-3310fps @ 26.3 gr
Today with the same brass, primer, powder, 2.250 OAL, & 55 gr Vmax & same rifle, out of 45 rounds, I had 1 FTE, 6 blown primers, and about 8 with very pronounced ejector marks on the case head. I've never seen either of my ARs leave an ejector (not extractor) mark before but admittedly I never looked twice at brass before I started reloading. Going back through my 2x fired stuff there are no ejector marks.
At 25.3 gr I was at 3486 fps. I should have just stopped right there. Either Chrony wasn't telling the truth or something was up. But they functioned fine & first 3 cases looked good, so I kept going.
Next was 25.7 gr & got 3500 fps but the Chrony only read 2 all day at that charge so that average is only 2 data points.
Last was 26.1 gr & got 3540 fps.
I was firing in groups of 3. 3 shots at one charge, then 3 at the next, then 3 at the next. For the first set, all rounds functioned fine and all brass looked good upon inspection. It wasn't until later on I started seeing issues.
I took these larger steps between charge weights because I figured I already know the slightly heavier (2/10) 55gr FMJBT is fine in this rifle up to 26.3 gr - probably beyond that - just never tried. Since I was staying under that, no big deal right? Bad assumption.
So when I got home I inspected the cases with the blown primers. I figured maybe these were some I cut the crimp out with a chamfer tool before I got the super swage. Sadly, that wasn't the case & these all went thru the SS - no cutting around the primer pocket. That's when I noticed the ejector marks.
All I can figure is I have the bullets set too deep which is spiking the pressure. I'm seating .030 longer, but the Vmax is .078 longer than the FMJBT. That put the base of the bullet almost .050 deeper in the case. Would that alone account for the large rise in speed and pressure?
BTW, 25.3 gr wasn't my start with the Vmax - that's just where I left off & started back today. Lower loads in the work up showed no signs of anything out of the ordinary but i didn't run them over the chrony. That would have probably alerted me a lot sooner that something was wrong. I'm not crimping these either - the FMJ I do, but none of the Vmax have seen the FCD. All charges were trickled on a beam scale.
Anyway, I'll start over and go with the 2.260 mag length and expect to stop a lot shorter in charge weight than I can with FMJBT due to the increased length. One thing about the Vmax - it really seemed to like the speed. Most groups were under an inch, several were at/under .8"
Below - from left to right - two 26.3 gr w/FMJBT, four with blown primers and pronounced ejector and extractor marks, two at highest charge with no pressure signs. Excuse the sharpie - that's how I tell what was what afterward. Blue=25.3 gr, green=25.7 gr, black=26.1 gr.
Same order, standing up.
I've run LC brass trimmed to 1.751-1.752, CCI #41, H335, OAL 2.220 & Hornady 55 gr FMJBT up to 26.3gr with no issues. Out of a 20" 1:9 AR they go right around 3300-3310fps @ 26.3 gr
Today with the same brass, primer, powder, 2.250 OAL, & 55 gr Vmax & same rifle, out of 45 rounds, I had 1 FTE, 6 blown primers, and about 8 with very pronounced ejector marks on the case head. I've never seen either of my ARs leave an ejector (not extractor) mark before but admittedly I never looked twice at brass before I started reloading. Going back through my 2x fired stuff there are no ejector marks.
At 25.3 gr I was at 3486 fps. I should have just stopped right there. Either Chrony wasn't telling the truth or something was up. But they functioned fine & first 3 cases looked good, so I kept going.
Next was 25.7 gr & got 3500 fps but the Chrony only read 2 all day at that charge so that average is only 2 data points.
Last was 26.1 gr & got 3540 fps.
I was firing in groups of 3. 3 shots at one charge, then 3 at the next, then 3 at the next. For the first set, all rounds functioned fine and all brass looked good upon inspection. It wasn't until later on I started seeing issues.
I took these larger steps between charge weights because I figured I already know the slightly heavier (2/10) 55gr FMJBT is fine in this rifle up to 26.3 gr - probably beyond that - just never tried. Since I was staying under that, no big deal right? Bad assumption.
So when I got home I inspected the cases with the blown primers. I figured maybe these were some I cut the crimp out with a chamfer tool before I got the super swage. Sadly, that wasn't the case & these all went thru the SS - no cutting around the primer pocket. That's when I noticed the ejector marks.
All I can figure is I have the bullets set too deep which is spiking the pressure. I'm seating .030 longer, but the Vmax is .078 longer than the FMJBT. That put the base of the bullet almost .050 deeper in the case. Would that alone account for the large rise in speed and pressure?
BTW, 25.3 gr wasn't my start with the Vmax - that's just where I left off & started back today. Lower loads in the work up showed no signs of anything out of the ordinary but i didn't run them over the chrony. That would have probably alerted me a lot sooner that something was wrong. I'm not crimping these either - the FMJ I do, but none of the Vmax have seen the FCD. All charges were trickled on a beam scale.
Anyway, I'll start over and go with the 2.260 mag length and expect to stop a lot shorter in charge weight than I can with FMJBT due to the increased length. One thing about the Vmax - it really seemed to like the speed. Most groups were under an inch, several were at/under .8"
Below - from left to right - two 26.3 gr w/FMJBT, four with blown primers and pronounced ejector and extractor marks, two at highest charge with no pressure signs. Excuse the sharpie - that's how I tell what was what afterward. Blue=25.3 gr, green=25.7 gr, black=26.1 gr.
Same order, standing up.