Not been discontinued, but are unavailable from time to time depending on priorities and backlog at Sierra.  The answer is to buy them and stock up when they are available. I try to keep about 5,000 on hand because it may be a few years until they become available again. Much like other reloading components. 

During the last shortage of SMK 174 grainers immediately post-Covid I called Sierra to inquire as to whether there would be another run soon.  The rep told me that calls like this were very helpful in determining which bullets outside of .308 caliber would be produced next and before long SMK 174 gr. bullets were back on the shelves.  I stocked up too and now that my current rate of expenditure has declined what I have on hand should last me years.  

LeHigh .311" 180 gr. solid copper target bullets appear to be back in production too of which I use to reload in my pre MkVII chambered M.L.E.  Loaded long I've had very good results with them, going from 4 moa with 150 gr. Hornady SST to 2 moa with the LeHighs.

Now I see brass is OOS...I have 500 between Remington & Winchester. I was gonna load up on PPU, but distributor shows OOS. 

I’m finding the PPU brass does not last as long as the WWII military brass.  I’ve been tracking the number of reloads on it. I get signs of web thinning after the second partial length (P/L) resize on about 5% of the cases. That’s after 8 to 10 neck sizings between P/L resizing.  

I’m attempting to P/L size the case so that the bolt locks with very light finger pressure, but on firing the cartridge after the second P/L resize, the “white line of death” is beginning to show.  I doubt I’ll get much more than 20 reloads on these cases.  On the military brass, I would get 30 to 50 reloads. 

    I put the cordite in the garden and burn the brass - a pop or puff now and then. Decent boolitz , great for a mad minute !. 

Oh boy, I would be happy with 10 loads each. I can get discount PPU brass in bulk, so when they become available again I'll let everyone know. 

My new Remington brand cases stretched much less than the new Winchester cases. The shoulders on the Winchesters stretched .050 first firing. The Remington was 1/2 as much at .025". I suspect my WIN cases will be toast sooner than anything else. I am curious to see what PPU does. 

I'm actually excited again about load development with the 174 SMK and potentially some PPU! Been a while since that's happened. LOL

Yeah, we're all real happy for him! 

The trick of course is to neck size only, until there is resistance in locking the bolt.     

Someone here on this site had a good idea of expanding the necks in new cases then neck size back to .308-.309 ID just far enough down the neck to form a second shoulder which holds the case back against the bolt face on the first firing. I haven't tried it yet, will have to find the right size mandrel to expand the neck.  I'm betting it will significantly extend the life of the brass.

Neck cracks can be completely eliminated by annealing on each reload. F/L sizing is a sure way to get a short case life.  

Yeah, I only have 394

Now that I know I'll only get 15 to 20 reloads, I need a lot more brass.  I've checked a few of the usual places and don't see any availability yet. 

It was a sizeable investment for sure but I was at the peak of my earning years and took five years to accumulate everything.  It wasn't simply reduced income in retirement that I was concerned about interfering with my hobby but one never knew when some Leftist moron in government night consider regulating sales of components and/or ammunition.  I wanted to government-proof my hobby from the likes of O'Bammy who proposed bans on the sale of spent cases and military surplus ammunition.  Fortunately the bans never materialized but that won't stop the next Progressive clown that infests the White House.

You're doing it right!