The deal with the lengths is that since the 9 mm Parabellum (9 mm Luger, 9×19 mm; I’ll just use 9 mm here) has a maximum average pressure (MAP) of 35,000 psi and the .38 Super +P MAP is 36,500 psi, that's just not enough difference that normal shot-to-shot variations in maximum loads won't include both numbers. So if you have to load to the same COL as the 9 mm when using the same bullet, then you have no significant additional powder capacity in the .38 Super and you've pretty much neutralized any advantage the conversion might have had over simply leaving the gun in 9 mm.
My guess is your dad was actually aiming to land somewhere between the two performance levels by counting on being able to get flat nose and hollow point bullets seated at least somewhat longer than they are in 9 mm. This would likely have taken some modification to the feed ramp and chamber mouth in the Astra, as the 9 mm itself won’t usually feed square nose bullets reliably at full 9 mm COL. Since it’s a straight blow-back design, I expect he would have gone to stiffer recoil springs, as well.
If you are setting up to reload anyway, you can certainly experiment with length to see what the magazine tolerates by taking at least three spare cases and bullets to make a set of dummy rounds. In some instances you need to make enough to fill the magazine to really check things out. You can take the bullets back out later with an inexpensive inertial bullet puller to load them with primers and powder. It’s often useful to keep one dummy round to speed up resetting your sizing die for that particular bullet in the future (you slip the dummy into the press and adjust the seater down to touch it), and to compare any trouble-makers to.
To make the dummies, start by resizing the cases or getting new brass (see last paragraph). Put bullets in each one at the bullet maker’s recommended .38 Super COL, or else start at 1.280" COL if you don't know that number. Try to put them in the magazine. If they fit, put the magazine in the gun and see if manually cycling the gun will feed all without jamming. Assuming this doesn’t fly on the first pass, turn your seating die's setting screw in by about a quarter turn and try again. In most dies this will be within 10% or so of 0.010” deeper seating. In Lee dies, with their coarser thread, it will take only about 1/6 of a turn. Try the fit again. Keep nudging the bullets in until you find a COL that fits and feeds.
With luck, you'll end up with a working COL that is longer than works with that bullet in the 9 mm. For example, the Hornady 124 grain FMJ FP (flat point; I know, I know; that’s an oxymoron, but it's what they use) bullet has manual recommended seating depths of 1.200" in .38 Super, and 1.050" in 9 mm Luger. If you find you can load it to 1.160" for reliable feeding in your conversion, then you are ahead in powder space by 0.11" and can load it faster than 9 mm at the same pressure, or load it to 9 mm velocity at a lower-than-9 mm pressure to be easier on the gun.
The next trick will be adjusting powder charges recommended for the .38 Super. Reducing the volume the powder burns in raises pressure. If the manual gives you a particular charge weight and COL, but you find you are having to use a shorter COL, you'll need to reduce the powder charge. In a straight wall case over small powder space changes, you will find it very close to true that the ratio of the reduced space to the powder space of the original load in the manual raised to the power 0.625 will give you the a charge weight multiplier to find the charge that maintains the same pressure with that same powder.
Plug the primer pocket of an empty case and weigh it. Fill it with water level with case mouth with no meniscus, no bubbles inside, and no drops on the outside. Weigh it again, then subtract the empty case weight. This gives you what is called case water overflow capacity (even though it doesn't actually overflow; the use of "overflow" is just to distinguish the term from case water capacity, which refers only to the weight of water that would fit under the bullet). You now calculate the volume of water displaced by the bullet base to subtract is to get case water capacity under the bullet. A short cut that will give you the result in grains, same as your scale, is to square the bullet diameter then multiply it by 197.2, then multiply that result by the bullet seating depth. This gives you the water weight displaced by a flat base bullet at that seating depth. Subtract it from the case water overflow capacity to see how many grains of case water capacity you have for powder.
To find seating depth, use the length of the case and the length of your bullet:
1.) Seating Depth = Case Length + Bullet Length - COL
Your case length is 0.895”. You bullet length is 0.553”. Your experimentally determined COL is 1.160”. The Hornady manual’s COL is 1.200”. Let us further suppose your measured case water overflow capacity is 18.0 grains of water.
Seating depths, from 1.), above, are:
For the manual load’s COL:
0.895” + 0.553” – 1.200” = 0.248”
For your COL
0.895” + 0.553” – 1.16” = 0.288”
0.355² × 197.2 × 0.248” = 6.16 grains of water displacement in the manual load
0.355² × 197.2 × 0.288” = 7.16 grains of water displacement for your load
(Note: You may multiply the difference in COL’s by .355^2 × 197.2 to get the displacement difference directly).
Now get the two case water capacities:
18.0 grains – 6.16 grains = 11.84 grains case water capacity of the load in the manual.
18.0 grains – 7.16 grains = 10.84 grains case water capacity of the load in the manual.
Now find the load adjustment factor:
(10.84 / 11.84) ^ 0.625 = 0.946
Now let’s use that:
The manual’s starting load of Alliant Power Pistol for this bullet seated to 1.200” COL is 5.5 grains. To find your adjusted starting load:
5.5 grains × 0.946 = 5.2 grains for your starting load
The manual’s maximum load of Alliant Power Pistol for this bullet seated to 1.200” COL is 7.1 grains. To find your adjusted maximum load:
7.1 grains × 0.946 = 6.7 grains for your maximum load
If you are feeling ambitious, you can buy a copy of the QuickLOAD software and use it to advise you of a more precise adjustment.
On brass: If you can't find a commercial load that fits, you can just buy new brass that's never been loaded to start from scratch. Small quantities are available from Midway and other reloading supply vendors. 500 and up are available direct from Starline. I recommend Starline as a brand, in particular, as I have always found their brass to have tighter weight and dimensional tolerances than most domestic brands, that their brass quality lasts through more reloadings than most—like old Winchester brass did—and it is also usually a little less expensive than other makes. In this case, I would get their .38 Super +P brass
, which they claim is made of heavy duty brass for the higher pressures. That will help you get to maximum pressure without issues. I would avoid the nickel-plated versions of any brand of brass as the nickel can eventually flake off and score your sizing die.