Great range results, unexpected velocity-- Pressure?

[BringBackThe80s]

I'm working up a .243 Winchester load for our compact/youth Ruger American (gen 1), using Hornady 90 grain CX loaded at 2.792" COAL (thirty thou off the lands) in twice-fired Norma brass, over H4350 and Federal GM210M primers. Hornady's manual lists 39.2 grains starting around 2800 fps and 45.0 grains max at ~3200 fps.

I was without a chronograph during initial pressure testing, so I worked up in 1-grain increments from 39.2-42.2, then in 0.2 grain increments from there. At 44.8 grains, there was a shiny spot (though not a defined circle) that might have been the hint of an ejector swipe. At 45.0 grains, there was an identifiable, though not severe, ejector swipe. So I decided to subtract a grain and set 44.0 grains as my working max.

Today I came back and shot 7 groups round-robin, moving from 42.8 to 44.0 in 0.2 grain increments, until a thunder shower stopped the party at 3 shots per group. The seven groups averaged 0.75 MOA, with only one of them edging over 1 MOA, and all seven together make a 1.55-MOA, 21-shot group. Consistent with the initial ladder/pressure test, the 4 charge weights from 43.4 to 44.0 grains all shot to the same vertical point, forming a 1 MOA 12-shot composite, with 43.8 and 44.0 each less than a half MOA. Neither thee bolt nor the brass showed any signs of pressure.

But today I had a chronograph and was surprised by the results.
- Good news: 43.8 grains ES=11, SD=4.5. And 44.0 grains ES=6, SD=3.1.
- Good news... or bad news: Velocity at 44.0 grains was 3130 fps out of an 18" barrel.

My guestimate for velocity loss due to short barrel had me expecting 2900-2950 fps. Instead, speeds were the same as if I was shooting a 24" barrel. Gordon's projectile library does not include the 90 CX, but my improvised model in GRT predicted 2935 fps. I like the idea of shooting 200 fps faster than I expected, changing this compact, 7lbs-total rig into a legitimate 500-yard deer rifle. But speed ain't free, pressure is the price, and I'd prefer to keep my brass, rifle, and face intact and not blended together.

So, does anybody have Quickload with a 90 CX in its library, or other tools or knowledge to estimate the PSI needed for H4350 to spit a 90 grain monolithic out an 18" barrel at 3130fps?

 

[BringBackThe80s]

For what it's worth, the chamber on this rifle is VERY tight. The bolt is stiff on closing some factory ammo, but then after firing it the bolt lifts easily and even closes easily if I put the fired case back into the rifle--indicating that fireforming shrank (not expanded) the case.

All fired/unworked brass slides freely in and out of the chamber. Resizing fired cased works the case sides a little, but that pushes the shoulders forward/longer by 1-3 thousandths longer than the shoulder/headspacing they have after fireforming. If I relube the cases and run them through the die a second time with gusto, I can get the shoulder back to its fire-formed length or maybe a thou longer, allowing reloadeds to cycle okay but with absolutely no wiggle room in the chamber.

I fired about 150 factory rounds before I got a die set and started reloading for this rifle and identified what was going on. It seems to me that the only real danger would be if closing/camming the bolt "sized" a case in the chamber, so that the neck grew long enough to hit the throat transition and crimp the case mouth, spiking pressure. Accordingly, I've run a borescope down from the muzzle and confirmed that both fired cases and resized but untrimmed cases are not getting too close to the neck/throat transition. (The only other risk is dirt, etc., preventing bolt closure on loaded ammo. I'll deal with that if/when it happens.)

So I don't think the tight chamber is a safety issue, and I suspect it may be one reason this rifle shoots almost everything quite consistently. But I suspect the smaller chamber dimensions reduce effective case capacity, which contributes to higher pressures and velocity above average for a given charge weight.

I'm also think that the tight chamber means that the inability of cases to expand during firing means that yhevrisk of case head separation is significantly reduced.

Any thoughts, experience, or suggestions on working with a very tight chamber?

 

[BringBackThe80s]

The primer pockets of the six cases that fired 43.8 and 44.0 grains all feel tight and are holding primers securely.

If this Norma brass stays unmarred and with good primer pockets, would you feel okay continuing to shoot the this recipe at 43.8 or 44.0 grains (1-1.2 grains below max and 82 thousandths longer than the Hornady manual's mag-friendly recipe)?

 

[menhaden_man]

My son has the same rifle, we’ve been using 90 gr HotCor and StaBall getting just shy of 3,000 fps. He shot a nice buck with it last fall, dumped it where it stood by the bullet (recovered on offside shoulder) shed all the lead… good shot placement but got me thinking about trying an 80 gr TTSX since the cup/core probably ruin meat at those speeds (mostly shots < 50 yds).

We loaded some of the ttsx last night with Superformance, expecting 3100-3200 fps but I’ll let you know.

I checked my Hornady book (10th edition) which doesn’t have the CX… but the lighter 80 gr GMX has max charge of H4350 at 3100 from a 24” bbl so can see why you’re scraping your head.

 

[BringBackThe80s]

My son has the same rifle, ... He shot a nice buck with it last fall, dumped it where it stood by the bullet (recovered on offside shoulder) shed all the lead… got me thinking about trying an 80 gr TTSX since the cup/core probably ruin meat at those speeds (mostly shots < 50 yds).

Congrats to your son! I got this one in 2021 for my three boys to use, but it's turned out to be more consistently accurate than my Tikka T3x Lite. It's a handy woods rifle. I shot a doe at 60 yards that fall using 100 grain Norma Whitetail ammo. She was facing straight toward me, so shot at the base of the neck. That bullet took out vitals but continued straight on through her end-to-end, exiting out her butt. She dropped like a rock, of course.

There was fragmentation and bloodshot tissue along a notable portion of the ribs under one of the backstraps, so I discarded a fair bit of meat. That was one of the kills that made me decide to switch to handloading copper for big game.

 

[menhaden_man]

Congrats to your son! I got this one in 2021 for my three boys to use, but it's turned out to be more consistently accurate than my Tikka T3x Lite. It's a handy woods rifle. I shot a doe at 60 yards that fall using 100 grain Norma Whitetail ammo. She was facing straight toward me, so shot at the base of the neck. That bullet took out vitals but continued straight on through her end-to-end, exiting out her butt. She dropped like a rock, of course.

There was fragmentation and bloodshot tissue along a notable portion of the ribs under one of the backstraps, so I discarded a fair bit of meat. That was one of the kills that made me decide to switch to handloading copper for big game.

Check out cavity back bullets, we’ve had decent luck with the 118 and 108 MKZ in a Grendel. They’ve got 243/6mm offerings just haven’t tried them out.

 

[06 SB]

Velocity is not free. Only one thing produces more velocity and that is pressure. If you are getting that high of a MV then you are over pressure. By how much is the big question. My latest project has been a 338Fed with Barnes 185 TTSX. According to Barnes data I max load of 49.0gr AA2230 gets me 2750MV (corrected for barrel length). I got that at 47.0. My load development as I have aged is to end at max published charge or when I reach the expected MV. A few extra FPS is not going to change the outcome of a hunt and is not worth risking my face.

 

[Q_Sertorius]

The Hodgdon website seems a fairly safe bet to cross check the load. It is not the exact same bullet and I am assuming you meant to write "H4350" vice "H435". SAAMI max for a .243 is 60k psi (51386 CUP).



If you are not seeing pressure signs, I would be comfortable with the results you are getting.

My estimate for an 18" barrel would be around 3000 FPS for this Hodgdon load (approximately 25-30 FPS less per inch of barrel length), but I have been pleasantly surprised by short barrel performance before. For example, my Hornady 6.5 Grendel 123-grain SST and ELDM factory ammo lists 2580 FPS on the box (with 24" test barrel). My Garmin shows me consistently getting 2509 FPS from an 18" Pac-Nor barrel. I would normally expect around 2400 FPS for this ammo from an 18” barrel.

The other question I have though is how large is your sample size? It sounds like you are shooting 3-round groups? Are you comfortable that you are shooting enough of a sample size to see pressure signs and consistent results now?

 

[BringBackThe80s]

The Hodgdon website seems a fairly safe bet to cross check the load. It is not the exact same bullet and I am assuming you meant to write "H4350" vice "H435". SAAMI max for a .243 is 60k psi (51386 CUP).

View attachment 914571

If you are not seeing pressure signs, I would be comfortable with the results you are getting.

My estimate for an 18" barrel would be around 3000 FPS for this Hodgdon load (approximately 25 FPS less per inch of barrel length), but I have been pleasantly surprised by short barrel performance before. For example, my Hornady 6.5 Grendel 123-grain SST and ELDM factory ammo lists 2580 FPS on the box (with 24" test barrel). My Garmin shows me consistently getting 2509 FPS from an 18" Pac-Nor barrel.

The other question I have though is how large is your sample size? It sounds like you are shooting 3-round groups? Are you comfortable that you are shooting enough of a sample size to see pressure signs and consistent results now?

Thanks for the input.

Regarding the sample size, yes, I think the velocities are statistically valid and representative. Velocities on the other 6 groups fell along the same the charge/velocity curve indicated by the 44.0 grain velocity. I weighed all the loads to within 2 hundredths of a grain (+/- 0.02 gn), and the chrono showed SDs of 3-4 fps. And the chrono reported expected velocities on a known load from a different rifle during the same range trip. It might be off by a normal 10 fps margin of error, but not by 100-200 fps.

 

[hickswr]

Mono bullets act differently than traditional bullets.

I’m running a 90gr hammer hunter in a 18” 6 creed @3160 with zero pressure signs.

3280 out of a 22” barrel.

 

[BringBackThe80s]

Mono bullets act differently than traditional bullets.

I’m running a 90gr hammer hunter in a 18” 6 creed @3160 with zero pressure signs.

3280 out of a 22” barrel.

Awesome! That's sounds very similar to what I am seeing. What powder are you using?

If it's a single base extruded, then the major factors (bore, case capacity, basic powder type, bullet type and weight, barrel length, and peak velocity) are all analogous to my setup and results.

 

[hickswr]

Awesome! That's sounds very similar to what I am seeing. What powder are you using?

If it's a single base extruded, then the major factors (bore, case capacity, basic powder type, bullet type and weight, barrel length, and peak velocity) are all analogous to my setup and results.

RL15.5 & H4350, both yield similar results.

Lapua brass, 450s

 

[BringBackThe80s]

RL15.5 & H4350, both yield similar results.

Lapua brass, 450s

Yeah, that is definitely pretty analogous to my rig and components. 6CM has a couple grains less case capacity than .243, but my rifle's overly tight chamber probably offsets that a bit, plus a 90 grain monolothic eats into the .243's powder column more than it does in a 6CM.

That does make me feel more comfortable about shooting 43.8 and 44.0 grains more as long as no pressure signs appear.

 

[TaperPin]

I’d mark a few cases and see how many reloads you can get before the ring shows up at the case head signaling thinning and partial separation. If cases last less than 5 it’s a hot load. That or an early loose primer pockets are what the pressure guidelines are there to prevent.

Some rifles are faster than others and while pressure does produce velocity, the pressure curve is affected by a number of things from throat wear to bore diameter. Even bullets are different diameters.