It's been about 3 years since I looked into the idea of making a kit to convert a Ithaca 37 to a China lake model with the duck bill. My cousin and I are looking into it again. We have the  duck bill shot spreader in cad and have been making some small changes to our design to make it as close as we can to the pics of the originals. . I'm hoping to make 1 to test and then I'd like to make a small run of them if it holds up to use, along with the clamps and the magazine tubes.  I probably won't be able to until around spring of 2017.

I'm trying to determine what I should make it out of . I was thinking either 4140 or s7, with the s7 being the preferred but much more expensive option . Is there any reason I should not go with 4140? Any metal people here with any other steels that might be good for this ( excluding stainless please chime in )


Pics of what we have been working on

Do you have the capability to heat treat the part?  I assume you do, if you are considering a tool steel.  

I would do a first run with 4140 or 4340 heat treated to a final hardness around 40 HRC.  Do some testing to see if they hold up, then move from there.

Frankly even doing the first parts with easily available HTSR 4140 bar (around 28 HRC) will probably last long enough to give you a good test and an idea if that's hard enough, you need a little more, or a lot more.

Yes I have a friend that can heat treat at his work . We're still working out the details , hopefully w can get somthing mad by April of 2017. I still need to Oder th new barrel for my shotgun to put it on.

It's relatively low velocity, low pressure, soft lead. This may be an instance where extra strength may be a weakness as it can facilitate cracking if not done right. Look at modern choke tubes by comparison, which can go for tens of thousands of rounds without problem.

I'm not an engineer, but I am a machinist who deals with exotic metals on a daily basis and I'd imagine an alloy like 4140 with a decent heat treating would hold up marvelously for this task. Using something like an S7 may get you slightly better wear resistance but at what cost and to what benefit? 150k rounds instead of 120k? But then you also have the issue of taking a finish, which it won't do as well as 4140; so it might function slightly better but will have trouble looking the part. Add in that an ordnance grade steel is more historically accurate and it seems to me like an alloy like 4140 is a better choice. I may be mistaken so don't quote me on this, but I believe I read once that 4350 was the standard ordnance steel used since WWII for US made small arms unless the material was otherwise specified.

I'd vote go with 4140 and I doubt you'll have any problems if heat treated properly for the task.

4140 is fine for what your after with proper post machining heat treat and/or cryo treat.  4350 would be a step up. S7 is just plain overkill for bragging rights.

Just remember that the weak link in this conversion is the mounting point to the OEM barrel and considering this I would mark the barrel after completion as "NOT FOR STEEL SHOT".