>UPDATE: I also found that I had been using magnum primers, when you are only supposed to use the regular large pistol primers for HP-38. So it must have been a combination of longer length bullet due to being solid copper, and using a magnum primer.
If you used jacketed data for a monolithic bullet, you have your answer right there.
Monolithic bullets require their own loading data, and the charges are a lot less. The magnum primer probably had nothing to do with it--you may have simply used the wrong load data.
Monolithic and frangible bullets can not be loaded like standard bullets.
Attached is the Hodgdon list showing the monolithic and frangible bullets they have in their load data.
My favorite are:
1) Recoil in excess of a factory round with the same bullet weight
2) For semi-autos: case being thrown further or in a different direction that a factory round with the same bullet weight
3) Pressure ring comparison: above the extractor groove by about 1/8-1/4" you will find a pressure bulge (mostly where the web ends and there is a pressure concentrator (can't think of proper term). When I fire factory rounds, I find the pressure ring is pretty consistent. If I then load virgin cases of the same head stamp, I can measure the increase in the ring as the loads go up and I stop as soon as I get a pressure rind equal to the factory rounds. I may be stopping a bit premature, but that is fine.
For example, a .38 Spl may have a 0.379" diameter before firing. After firing a factory round, the pressure ring might be 0.3830". As I work up the load, I might get 0.3800", 0.3806", 0.3809", 0.3820", 0.3826", and 0.3829". In reality, I might be able to go up higher, but in reality, I have most likely already stopped as accuracy was going down and I have no need for max or near-max load.
4) Sudden change in primer appearance.
For a further writeup (though a lot has to do with bottleneck cases), I quote from: https://www.shootersforum.com/handlo...ure-signs.html
on Pressure Signs by unclenick
Load manuals and experienced reloaders frequently advise working a load up in small charge increments while watching for pressure signs. Many beginners are unaware of all the signs, so, below is a partial list of pressure signs, along with alternate causes of the signs where I am aware of them. No one particular sign can be counted on to work in all guns nor even with any particular set of load components all the time. There is also no way to tell which sign will show up first in your gun and with your load combinations, so you have to learn to watch for them all.
1.Case bulging, particularly near an unsupported part of the head.
2.Case crack along side (may mean excess pressure, but may mean brittle, defective, draw mark scored, or worn out brass).
3.Case head expansion (CHE; may mean high pressure, may mean nothing in isolated case).
4.Case head separation (may mean high pressure, but may mean excess headspace or worn out brass).
5.Case splits in body in under 10 reloads-back loads down at least 2% (can also be due to ammonia vapor exposure or a brass defect in an individual case).
6.Case mouth split (may mean high pressure, but more often means case needed neck annealing).
7.Case pressure ring expansion (PRE; not much more reliable than case head expansion but may mean pressure is excessive).
8.Case primer pockets getting loose in five reloadings or fewer.
9.Case excessive stretching (this is actually visible pressure ring area stretching which may be due to excess pressure or to excess headspace).
10.Extractor marks appear on case head in semi-auto rifle after incrementing powder charge up (may be high pressure or bad timing or an extractor standing proud on the bolt face).
11.Fired case won’t fit back into chamber.
12.Gas leak (see Primer Leaking, below).
13.Groups start to open up at or beyond a suspected maximum load pressure.
14.Hard bolt lift.
15.Incipient case head separations (partial case head separation).
16.Incremental increase in powder charge results in lower velocity or at least in no increase in velocity (may also mean uneven bolt lug contact being forced to touch down on both sides; watch for stringing on the bolt lug axis as additional symptom of this). 17.Primer blown (primer falls out when gun is opened; same as loose primer pocket). 18.Primer cratering (may mean high pressure, or it may mean a worn firing pin or firing pin tunnel, or may mean you have a new production Remington bolt with chamfered firing pin tunnel).
19.Primer flattening (may mean high pressure, or may mean long headspace; some loads always make flat primers; softer primer cups (Federal) flatten more easily than harder ones (CCI), so it also can mean nothing at all).
20.Primer mushrooming (may mean high pressure, or may mean long headspace).
21.Primer piercing (may mean high pressure or may mean incorrect firing pin protrusion or incorrect firing pin nose shape).
22.Primer leaking around primer pocket (may mean high pressure, may mean loose primer pocket in case, may mean damaged primer was inserted, may mean primer backed out too far during firing, which excessive chamber headspace makes possible).
23.Short case life -back load off at least 2% (under 10 reloads in non-self-loaders or with military brass in self-loaders, 6 or less in self-loaders with commercial brass).
24.Sticky or hard case extraction (especially in revolvers this is a positive sign to knock the powder charge down at least 5%).
25.Torn case rim (from hard extraction).
26.Primer pocket expansion (PPE; this is likely no more accurate than CHE (3., above), but is a more sensitive measure for those with tools that can measure the inside diameter of a primer pocket repeatably to the nearest ten-thousandth of an inch).
27.Ejector and extractor impressions on the case head (can also be due to ejector and extractor fit problems).
28.Increase in required trimming frequency (this is a sudden increase in case length growth per load cycle; it can be caused by excess pressure, but can also be a sign of increasing head space due to some other problem. It is especially common as a pressure sign in lever action guns because the greater span from bolt face to rear lug allows more steel stretch when pressure gets excessive.) 29.Increasing apparent headspace (this means the cases are coming out longer, including from casehead to shoulder. It can mean bolt lug setback, which is usually an extreme pressure sign. It can also mean a loose barrel or an improperly set Savage barrel. Whatever the cause, the gun should go straight to the gunsmith for inspection.)
30.Gas or Flame Cutting of revolver top strap. (Can also be due to excessive barrel/cylinder gap that needs correction.)
31.Gas cutting of rifle bolt face by gas leaks around primer pocket. (Can also be result of occasional leaks from normal rounds firing, as is observed in many military gun bolts.)
32.Velocities at 15 ft higher than manual maximum load velocity for same powder and barrel length.
For example: one fellow using a .243 Win load one charge increment below the manual maximum got velocity 200 fps higher than the manual claimed for its maximum load's velocity. His single-shot action was popping open at every shot. With QuickLOAD we were able to calculate he had about 77,000 psi. An alternate explanation, if everything else is normal, is that your chronograph readings are incorrect. It is not uncommon to get high readings due to muzzle blast when the chronograph is too close to the gun. I recommend 15 feet minimum, since that is what the manual authors typically use. Some big magnum rifles need even more distance.