There is every possibility of achieving M50 result by hand mixing.
Step 1: Mix design for the said grade to be prepared
Step 2: Hand mixing involving turning over the dry mixture from one end of the mixing tray or platform to the other end completely so as to get the uniform mixing of the concrete ingredients.
Step 3: Adding water as per the design mix part by part to this dry mixed concrete again starting from one end of the tray to the other end, this process to be repeated for at least 4 to 5 times for best results till you see a homogenous concrete mix.
Step 4: Proper mixing of the concrete from one end of the tray to the other end is the most critical part of any hand mixing process.Adequate strength requirement is one of the major properties of concrete and this strength depends on the consistency of the mixing.
It is important to know the minimum number of times required in turning concrete over from one end of the mixing tray to the other, in order to produce concrete uniform in composition and of satisfactory strength.
It is very necessary to have a good understanding of the relationship between the degree of mixing by hand and the compressive strengths of concrete.
Most literature recommends mixing the concrete until the mixture is consistent and uniform in nature based on physical observation.
This recommendation may not be completely a good measure of proper mixing since observers have different views on when concrete is consistent and uniform in nature.
Hope that I have given justice to your question.
I have obtained desired strength of M:40 grade while doing mix design trial mixes, hence am able to say with confidence, however would suggest you to first carry out trial mix before venturing out on the production. Thanks....