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My first 7.62x39 AR

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18 hours ago, 98Z5V said:

I LOVE Model 52s!!

Beautiful guns they are; I wish I pulled this out for comparison earlier. I know it's a bit nitpicky, but I might have gone with a 12" handguard to match the profile better. 

I had to go outside and scare off a squirrel, so I figured it was a good time to test the shot timer. It sure was a different experience shooting on command. Not too surprisingly it picked up the shots just fine even with the low input volume. Plenty of programming work left to do, but it is nice to see it functioning in a real world test. 


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No real update today. A ton of strange issues writing to the file system in the shot timer. It usually works ok; but I need to work out an initialization function for first time programming/first boot. I have the settings file reading and writing from the web interface. It's a tiny bit of copy/paste, and variable modification to do the same for the shot timer programming web page. The shot time data already writes to a file, but I haven't worked out the ability to download it; outside of a copy/paste of the webpage.

Many updates in the electronics side of things; but it just made me order more parts, again. Nothing expensive; I think the final unit is still around $20 in component cost. I did a tiny bit of optimization in the shot recording code. I can get 2-3ms audio resolution while still running the display, WiFi, and webserver functions so far. If I turn off the big ones, I should be well below 1ms resolution; and there can be further tuning beyond that. There is very little bounce with the piezo pickup, with an appropriate threshold set, it does not pickup any echo noise at all.

One of the new electronic scales showed up; but it is worse than the first one. Even after warming up and calibration it can't measure small amounts. Interestingly the packaging has cutouts for smaller calibration weights; but none are included or usable in the calibration process. Should have the load cell ADC by Friday if package tracking is accurate. I measured about 15 kernels before the balance of my Lee powder scale moved. I don't think the improvement in weight accuracy is necessary, but I will try to ensure consistency if possible.

This 7.62x39 build needs to be cleaned and broken down yet. Too cold outside to care right now.


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Still plugging away at the code but there have been a number of improvements since last night. Turns out some of my file writing woes were due to an undocumented oddity in the file mode commands. You can only seek a file in r+ mode (read+write); but not in w+ (write+read), or a+ (append+read). My programming to do list is shrinking rapidly, but I'm still not sure what day I'll have a fully functional unit. I will soon need to commit to a button layout, solder it, and mount it in the case. The only other thing I really haven't messed with is the battery power supply, and a charger powered by the USB port. I had used a standard USB battery bank for my outdoor testing the other day.

I ported the code to the new larger display microcontroller to work out the first time file initialization functions; turns out the display has a different driver chip, but it is mapped to the same pins. That only took a few minutes to figure out and resolve. Somewhat surprisingly the ADC wiring on the circuit board must be routed much more intelligently; the noise on the new board is about 1/3rd of what I was getting on the module without built in display.

On the electronics side, I had tried one of the silver piezo transducers I picked up from mouser; but couldn't solder to the silver surface no matter what I tried. Then the connection to the crystal element snapped off; so that's a no-go. Looks like they could be mechanically mounted with spring terminals, but that's not a good solution. I ordered some pre-wired 27mm piezo pickups which should have good low frequency response, and also fit perfectly inside the width of the enclosure. The buzzer had always been quiet; so I swapped that with a new unit also from Mouser. That was quite a bit louder but still not loud enough. Even with transistor amplification, it isn't cutting it in terms of volume. The other options are differential drive, or a charge pump; but a quick test of differential drive crashed the microcontroller. I bought a few active and passive buzzer modules which should be louder. If that's not enough, I'm going to have to find and disassemble a LiPo battery alarm that has a deafeningly loud buzzer.

Here is a look at the OSD for the shot timing function. I may have to slow down display updates in the future; but the progress bar currently shows a live view of sound amplitude. I still need to write some code to display timing data for a short period of time; but I don't want to clutter up the screen with nonsense either. 

One thing I was still curious about is; how many shots would reasonably be recorded in a single shot string (or shot timing program)? I currently record up to 127 shots to RAM as the maximum before dumping it to the filesystem. The only problem with a huge number here is RAM consumption. Writing shot data to the file takes time though too; so I want avoid any interruption to the shot timing capture process.



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Minor 7.62x39 update. I stopped by the local gun shop today for some odds and ends. I couldn't find a .30 caliber patch loop, but they did have a brass jag. I asked about online ammo purchases, and they offered to hand it over free of charge since I'm a regular customer. Sweet deal! Either they can order it for me, or I can order online and just stop by and pick it up. I guess I'll be able to stock up on 7.62x39 in brass at $0.40 a round after all.

I called green mountain about the barrel. They said the bore is between 0.300"-0.302", and the groves are 0.310"-0.312". Seems like I'll want to buy .311 projectiles for reloads. Interestingly the load data sheet that came with my dies explicitly warns that the minimum load is the max load for shooting .311s out of a .308 barrel. 

I'll probably get my build ready to fire later tonight. There are two snow storms coming through this weekend; so it's either now, or sometime next week before I'll be able to shoot again. 

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