Noob Grendel reloader, are Presure signs different for the Grendel vs the 6.8?

[Brag]

In the comings months, I plan to unload Factory loaded ammo, then the pile of Brass will call my name to start reloading them.

I'll work up many loads that are posted here, it seems the upper limit is well established for many projectiles. When working up loads(I'll start with reduced loads), I still look for pressure signs, and watch what the chrony is doing.

When loading for the 6.8, the loads start to get warm, Swipes defiantly show a hot load. I tend to use CCI400s for the 6.8, find a good load then switch to CCI#41's. Watching for primer flattening also shows the pressure is cooking. I do feel that pressure signs happen North of 55k, and maybe even 58k for the 6.8SPC.

52k seems the level for the 6.5G, and guys are watching for Primer pressure sign's and seem to get them at the 50k to 52k level. When using the same primer(and size, small), is the size of the case head affecting when pressure signs show up, or are they the result of peak pressure. Bolt thrust difference maybe?

All cartridges are different, and I'm a little behind the curve of Knowledge for the Grendel.

Thanks in advance!

[rasp65]

Brag The pressure signs should be about the same. Make sure your cases are sized properly, they should be cycled after sizing without bullet to make sure they are not too long. You will also want to measure your cartridges overall length. I outlined my method here http://www.65grendel.com/forum/showt...jump-amp-crimp Start low and work up. Good luck and good hunting.

[bwaites]

Trusting pressure signs in a gas gun is a short trip to disaster. Traditional pressure signs are disrupted by the bolt opening, among other things, so using them to help develop loads can get you in trouble quick.

Experience tells us that some loads will show no traditional pressure signs until they are WAY past the safe operating pressure. Some PROOF loads don't even show the traditional signs of pressure you would expect. If you see pressure signs, be VERY careful!

Traditional pressure signs were observed in bolt guns, where the cartridge was locked into place the entire time. The gas gun is starting to eject the cartridge within milliseconds of firing, and so those signs are often not seen.

For instance:



BUT...there was no swipe, and the primer looks pretty normal. Obviously, that load was too hot for that rifle, but the traditional swipe/flattened primer was not present.

[JASmith]

Bwaites -- nice description and picture.

Thanks for reminder that the AR15 bolt starts to open while the high pressure gas starts to leave. Sometimes before, depending on length of barrel and gas port location.

So, short of a pressure rig, how do we tell when the pressures are in that 50ksi range that we're pretty sure SAAMI is going to prescribe?

[bwaites]

Bwaites -- nice description and picture.

Thanks for reminder that the AR15 bolt starts to open while the high pressure gas starts to leave. Sometimes before, depending on length of barrel and gas port location.

So, short of a pressure rig, how do we tell when the pressures are in that 50ksi range that we're pretty sure SAAMI is going to prescribe?

Great question, and one which I'm not sure I'm qualified to answer! I look at all those things, but when I see one of them, I suspect I'm overpressure, when I see two I KNOW I am, and if I see 3 or more; well, I feel really lucky if nothings broke and I have all my body parts!

When I see one of those things, I back off and re-evaluate that load.

[LRRPF52]

I rely on the chrono first and foremost for signs of strange things happening. With increasing loads, I see a relatively predictable MV increase, then all of a sudden, erratic spikes and dips in velocity appear. That tells me something weird is going on, and I'm in unpredictable territory. I don't like the terms "unpredictable" and "50,000+ PSI" mixed together inches in front of my face...just me and a sense of self-preservation I guess.

With the brass pic above, it shows that you can have a "safe" load that still ruins brass in a gas gun due to dwell time and the gas system causing early unlocking, especially with the dwell time arrangements that are longer than the 20" barrel + rifle-length gas system.

[nincomp]

At the risk of sounding like a broken record, long, long ago, in the days before this forum was first destroyed... Bill Alexander pointed out what bwaites noted about the brass. The Lapua brass is essentially the same as the PPC brass that is routinely run up to ridculous pressures in benchrest bolt guns. The 50K pressure limit was set for long life of the AR15 rifles (especially the bolt). At least one AR was kaboomed when the reloader switched bullet types of the same weight without dropping reducing the charge. Luckily, the AR is a relatively forgiving rifle and can handle higher pressures for a little while before the bolt shears a lug or something more serious happens.
At least around 2005, the popular program "Quickload" had a glitch with predicting pressures for the 6.5 Grendel. It predicted 60+K pressures for loads were measured in a pressure barrel at 48K or so. I don't know if this problem was ever fixed.
To make a long story short, the only loads that you can be sure about, in an AR15, at least, are ones that have been pressure tested. Luckily, there is a lot more data online now from the powder companies. Don't forget to look at the Alexander Arms data as well.

[JASmith]

Nice summary nimcomp!

We are spoiled by having access to a number of well-proven exterior ballistics codes. So we get disappointed when a different calculator gives surprises. The cardinal rule is calibrate, calibrate, and calibrate again!

I have run QuickLoad against a number of published data sources and it appears that the major challenges pop up when using long bullets. I communicated with a Ramshot Ballistician about 18 months ago and he indicated the QL math model does not include the distance between the bullet base and the primer. This means that the bullet gets more of a kick from the primer than the model assumes when the base is real deep. Modeled pressures then get high real fast compared to the real world. He gave me some suggestions on how much to turn certain knobs and things got better. Results with the revised settings, however, were not enough better that I could feel confident that I'd get a good answer for that class of bullet when going to a different powder.

One could assume other short cartridges like the WSSM cartridges with long all-copper bullets seated to magazine length would exhibit similar anomalies.

You also make an excellent point that there's a lot more data to draw from -- so one can generally run QL against loads with the same bullet to get a better feel for whether you're getting garbage or something else.

Even then one needs to check against independent sources (calibrate...)

[mseric]

I do feel that pressure signs happen North of 55k, and maybe even 58k for the 6.8SPC.

Yes, North of 55K, way north of 55K. It seems that many handloaders and shooters believe that if a cartridge is rated at say 55K then a load just over 55K will show signs of high pressure. This is simply not the case. Tradition pressure signs like swipe usually do not show up until the brass case has reached it's pressure limits, these limits are well above 55K and even above 65K. The brass case does not know what SAAMI has listed as it's max pressure.
In an AR type rifle that has a properly running gas system and you see signs of swipe or really flat primers then you have gone well past 55k and are most likely into 70K range.

[fanofflyn]

I have also noticed that I get pressure signs earlier in load development with my piston gun than I do in the DI guns. I will usually develop loads first in the Piston, and then work them up in the DI guns to the max I developed in the piston rifle, but no further. I just feel safer that way, and I have yet to develop a pressure sign in the DI guns when I do this. However, that is my own piston gun, and I can't say that would work for anyone else.