If I had a $1000, what short term investment could I use to gain double the money within 6months?

To double your money in 6 months you would have to earn at an annual rate exceeding 144%. There are no short term investments that will earn at this rate. Only wild speculation (for example go to a casino and place your money on red or black at the roulette table), may allow you to double your money in that time frame, but you have a more than 50% chance you will lose your money at the roulette table, and probably a greater percentage of losing it at other gambling. As the Gurus say Investing is a marathon not a sprint.

To find out how fast you can double your money use the rule of 72. Divide the rate of return you get in one year into 72. For example at 8% money will double in 9 years (72/8 = 9).

That is a question we all would like answered. I don't think you will find one. Only kind of investment would be a hot stock, that is going to double its value.

Doubling $1000 in six months is a huge goal. Your best bet is the stock market which is hit and miss even for experienced investors.

If I were you and seriously want to double $1000 in six months, you have to think outside the box and take major risks.

1. Make six one month micro-loans at an extreme interest rate (150%). The breakdown would be as follows:
Month Loan Amount Interest earned
1 $1000 $125
2 $1125 $140.625
3 $1265.625 $158.2031
4 $1423.828 $177.9785
5 $1601.807 $200.2258
6 1802.032 $225.2541

At the end of six months you would have approximately $2027.287

Of course, you have to first find people willing to take this loan and get them to pay you back. And my guess is that anyone willing to take this loan would be a huge financial risk (i. e. likely never to repay the loan).

2. Enter a real estate deal with a group of similar minded people. Take 30 people with $1000 each, pool all funds for a total of $30,000. Find a property that can return $30,000 profit. Get a bank loan for the group to cover the purchase and renovation of the house with the group's $30,000 serving as collateral/extra funds. Sell the house and take your invested $1000 and your $1000 profit. Now you have $2000. This is also a stretch, but it is withing the realm of possibility. The contract and lawyer fees for the 30 separate investors would be cumbersome. And if your city is a slow market for real estate at the moment, this will be tough to do.